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Dimethyl mercury (DMHg) is commonly detected in the world's oceans, but little is known about the mechanisms responsible for DMHg degradation in natural waters or the products of this degradation. Similarly, the potential for the conversion of DMHg to monomethyl mercury (MMHg) under the acidic conditions commonly used to preserve samples for MMHg analysis has not been fully addressed. We provide evidence suggesting that DMHg in natural seawater is not readily photodegraded by sunlight as previously thought. Other experiments demonstrated that DMHg in seawater is, however, readily decomposed under acidic conditions, with MMHg as the predominant product. This facile conversion of DMHg to MMHg at low pH both necessitates an alternative preservation method to acidification for samples to be analyzed for MMHg when DMHg is present, and requires that data from previous studies of MMHg in seawater employing sample acidification be revisited in instances where appreciable DMHg concentrations were possible.