Surface measurements of ozone, aerosols (in 10 different size ranges), carbon monoxide, and methane are made during a ship cruise over the Indian Ocean from January 5 to February 3, 1996. These results have been used to study the variabilities in their distributions and mixing of the continental and pristine air masses over the tropical oceanic region. Concentrations of ozone, aerosols, and carbon monoxide show decreasing trends from coastal region to the open ocean region. During this cruise, influence of the continental polluted air from the South Asian region was observed, resulting in high levels of these species in the open ocean. Incidence of encountering extremely low levels of ozone has been observed near the equator. These low and high values of ozone, aerosols etc. are supported by the back trajectory analyses made using European Centre For Medium-Range Weather Forecasts (ECMWF) meteorological parameters. These results confirm the transport of the continental air mass down to the equator over the Indian Ocean. A simple photochemical box model has been used to estimate the ozone production potential (OPP). The discrepancies in the observed and estimated ozone diurnal variation suggest a need for better understanding of the processes. ¿ 1998 American Geophysical Union