The active lifetime of extracellular enzymes is a critical determinant of the effectiveness of enzyme production as a means for heterotrophic marine microbes to obtain organic substrates. Here, we report lifetimes of three classes of extracellular enzyme in Arctic seawater. We also investigated the relative importance of photochemical processes and particle-associated processes in inactivating extracellular enzymes. Enzyme inactivation in filtered seawater was slow, with apparent half-lives of enzyme activities on the order of hundreds of hours. The presence of particles (including cells) did not significantly change inactivation rates, suggesting that the long half-lives observed in filtered seawater were realistic for enzymes in unfiltered seawater. Phosphatase and leucine aminopeptidase were susceptible to photoinactivation, but only under high intensity UV-B and UV-C illumination; there was no evidence for increased inactivation rates under natural illumination at our study site in Ny Ålesund, Svalbard. Comparison of inactivation rates of commercially-obtained enzymes from non-marine sources with the extracellular enzymes naturally present in Arctic seawater suggests that the natural enzymes contain structural features that confer longer lifetimes, consistent with observations reported by others from a range of field sites that cell-free enzymes can contribute a substantial fraction of total hydrolytic activity in the water column.