We conducted a detailed investigation of the evolution of methyl bromide concentrations, degradation rates, and ventilation rates for 26 days in a naturally contained, warm-core eddy of the North Atlantic Ocean. This is the first study of the oceanic cycling of methyl bromide in a natural, contained system with a complete suite of supporting measurements of physical and chemical variables. Methyl bromide concentrations in the mixed layer ranged from 2.3 to 4.2 nmol m−3, degradation rates ranged from 0.1 to 0.9 nmol m−3 d−1, net sea-to-air exchange rates ranged from 0 to 0.5 nmol m−3 d−1, and net loss rates through the thermocline were less than 0.1 nmol m−3 d−1. From a mass balance for methyl bromide in the mixed layer, we calculated production rates ranging from <0.1 to 1.3 nmol m−3 d−1. The median of this range, 0.48 nmol m−3 d−1, is higher than the ∼0.15 nmol m−3 d−1 necessary to maintain the reported global oceanic emission of 56 Gg yr−1. This is reasonable, because our study area was supersaturated in methyl bromide, whereas the ocean as a whole is undersaturated.