As part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE), ship-based aerosol size distribution measurements have been carried out with a scanning electrical mobility spectrometer (SEMS) aboard the R/V Oceanus. The fast time resolution of these measurements illustrates some of the short-timescale variability of both the marine background aerosol and the anthropogenically influenced continental air masses. The resulting total number concentrations are largely in agreement with those from a collocated condensation nucleus counter (CNC). The aerosol size distributions provide characteristic signatures for different atmospheric conditions, showing low-concentration bimodal distributions in cleaner air masses and higher-concentration single-mode distributions in air masses with apparent recent continental influence. ¿ American Geophysical Union 1996