Measurements of air-sea gas transfer were made during the development of a large algal bloom that immediately followed in-situ iron enrichment in the equatorial Pacific Ocean. Gas transfer rates were determined from changes in the ratio of the deliberately released tracers sulphur hexafluoride and helium-3. This first application of the dual tracer technique in the open ocean produced estimates of the gas transfer rate that were similar in magnitude to those obtained previously from measurements in shelf seas. We found no evidence that gas transfer rates declined during the development of the algal bloom. Incorporation of the new open ocean measurements with other published dual tracer data gives a relationship between transfer velocity and wind speed that explains 82% of the total variance in the dataset.