We compare how sustainability loans issued by U.S. and European borrowers change valuations around a major amendment to global sustainability-contracting guidelines. Loose U.S. federal regulations help us disentangle the effects of public and private interventions in ESG performance. This private amendment increased the value of U.S. sustainability loans over conventional loans, and borrowers' environmental and social performance. European sustainability loans were not affected by the amendment, indicating that private interventions have little impact under strong government regulation. The amendment effect is stronger for U.S. firms without strict state climate rules and for those without European subsidiaries, the latter suggesting a regulatory spillover. Our findings highlight that private rule-setting can make up for lax or late public regulations in driving value-enhancing sustainability efforts.