Reactive gaseous mercury (RGHg), atmospheric elemental mercury (Hg0), and ozone (O3) along with other ancillary parameters have been measured simultaneously in the marine boundary layer (MBL) over the North Pacific Ocean during the 2002-IOC cruise between Osaka, Japan, and Honolulu, Hawaii. Atmospheric Hg0 concentrations varied from 1.6 to 4.7 ng m-3 with an average of 2.5 ng m-3 and did show significant variations along the cruise track with a slight diel cycle in Hg0 concentrations. RGHg concentrations varied from 0.15 to 92.4 pg m-3 with an average of 9.5 pg m-3. The data strongly suggest photochemical in situ formation of RGHg with increasing concentrations in the tropical regions concomitant to a dramatic decrease in O3 (from over 50 to less than 5 ppbv). A distinct diurnal variation in RGHg concentrations with maxima at midday suggests a photochemically driven oxidation of marine boundary layer Hg0. In the tropical area, enhanced RGHg formation and daily variation in O3 could both be related to reactive halogen chemistry. Flux calculations also emphasize the role of the tropical marine boundary layer in mercury cycling over the North Pacific Ocean.