The production mechanism of light nonmethane hydrocarbons (NMHC) in seawater was investigated during the North Atlantic atmospheric chemistry program (NATAC) in April and May 1991 in the European coastal seas and the North Atlantic. A significant alkene production occurred in the presence of light only. Under conditions of negligible NMHC emissions (low wind velocity) increasing hydrocarbon concentrations were observed during daytime, whereas the concentrations remained constant during night. NMHC formation experiments were carried out with seawater filled in quartz glass bottles and showed the same dependence of light. Experiments with differently pretreated seawater samples indicated that the presence of dissolved organic material (DOM) is also necessary for alkene production. We suggest a two-step production mechanism for alkenes: first DOM is released, probably from algae, then part of this material is photochemically transformed into alkenes. The production rates in the quartz glass bottles were comparable to the production rates in the ocean surface. This indicates that the processes occurring in the experimental setups represent the processes occurring in the field. Since the production-and emission rates were in the same range it can be concluded that the budget of light alkenes in the remote marine environment is determined by the production in seawater as the dominant source and the flux into the atmosphere as the main loss process.