Satellite measurements of the global distribution of aerosol and their effect on climate should be viewed in respect to a baseline aerosol. In this concept, concentration of fine mode aerosol particles is elevated above the baseline by man-made activities (smoke or urban pollution), while dust or sea-spray elevates the coarse mode. Using 1--3 years of measurements in 10 stations of the Aerosol Robotic network (AERONET) we develop a methodology and derive the optical thickness and properties of this baseline aerosol for the Pacific and Atlantic Oceans. Defined as the median for periods of stable optical thickness (standard deviation <0.02) during 2--6 days, the median baseline aerosol optical thickness over the Pacific Ocean is 0.052 at 500 nm with ¿ngst¿m exponent of 0.77, and 0.071 and 1.1 respectively, over the Atlantic Ocean. ¿ 2001 American Geophysical Union