Sustainability Bonds

A sustainability bond is a bond whose proceeds are used to finance or refinance a combination of environmental and social projects. A single issue might fund solar farms and flood defences alongside hospitals, schools and affordable housing. The bond is otherwise a normal debt security: investors receive the coupon and their principal back on schedule regardless of how the projects perform, and the credit rating is that of the issuer.

Sustainability bonds are the third member of the use-of-proceeds family, alongside green bonds and social bonds. They follow the Sustainability Bond Guidelines (SBG) published by the International Capital Market Association (ICMA), which apply the Green Bond Principles and the Social Bond Principles together. The name causes confusion with sustainability-linked bonds, which are a different structure: an SLB's money can be spent on anything, and its terms change if the issuer misses sustainability targets.

Imagine a charity jar at a shop counter with a label saying the money will go to two things: planting trees in the local park and running a youth club. Donors know the money will not pay the shop's electricity bill, and the shop posts a note each year saying how many trees were planted and how many children used the club.

A sustainability bond is a loan rather than a gift, but the idea is the same. The borrower promises that the money will go only to a published list of green and social projects, keeps track of it and reports back. Investors get repaid in full either way; the label tells them where their money is working.

ICMA first published the Sustainability Bond Guidelines in 2017, together with the Social Bond Principles, and updated them in 2018 and June 2021. The SBG are short because they work by reference: a sustainability bond should meet the four core components of both the GBP and the SBP. Those are:

• Use of proceeds, with each project falling in an eligible green or social category; some projects deliver both, such as energy-efficient social housing.
• A clear process for project evaluation and selection, including the target population for social projects.
• Management of proceeds, tracked and with unallocated balances disclosed.
• Reporting at least annually on allocation and impact.

Because the GBP and SBP were both updated in June 2025, the rules that sustainability bonds follow reflect those editions. Issuers are encouraged to publish a combined framework and obtain an external review, usually a second-party opinion, before issuance.

Sustainability bonds suit issuers whose spending spans both kinds of project: development banks, governments, municipalities, universities and companies with broad sustainability programmes. Development lenders use the format widely for funding tied to the UN Sustainable Development Goals, which cover both environmental and social aims. Sovereign issuers in emerging markets have used sustainability bonds to fund budgets that include climate adaptation and poverty reduction, often mapped to the Paris Agreement and to national development plans. Banks issue them to refinance portfolios of green mortgages and loans for financial inclusion.

In Islamic finance, the same idea appears as sustainability sukuk, which combine the green and social uses seen in green sukuk with a Sharia-compliant structure. The deal is sold through normal underwriting channels, with the framework referred to in the prospectus. Investors include pension funds, insurers and ESG bond funds following sustainable investing mandates. Much of the market is investment grade because many issuers are public bodies or large companies.

The main advantage for issuers is flexibility: one framework and one benchmark-sized bond can cover a wide spending programme, which helps liquidity in the secondary market. For investors the format offers exposure to both themes from a single issuer.

The criticisms mirror those of green and social bonds. A broad framework can make it harder to see how much money went to each category and whether projects are additional or simply refinance existing budgets. Social impact is hard to measure, and reporting quality varies. External reviews are paid for by the issuer. Sustainability bonds also sit outside the EU's European Green Bond Standard (Regulation (EU) 2023/2631), which covers environmentally sustainable uses only. Investors can judge a deal by reading its framework, second-party opinion and allocation reports, and by checking whether the issuer's wider strategy fits the label. In the end the bond's financial risk is the issuer's credit risk, as with any corporate bond or government bond.

• ICMA: Sustainability Bond Guidelines