The overall exchange coefficients expressed on a gas phase basis, Kg, for the transfer of H2S from water to air was measured in the laboratory by using radioactive H2S. The exchange coefficient showed a strong dependence on solution pH that was found to be related to enhancement by ionisation of H2S to HS-. At low pH values, where this enhancement has no effect, the value of Kg was 2.3¿10-2 cm s-1. At sea water pH, Kg increased to 0.13 cm s-1 leading to the conclusion that at pH<5 liquid phase resistance dominates the exchange while with increasing pH gas phase resistance becomes increasingly important.