Knowing the concentration of hydroxyl (OH) radicals is most important for the understanding of the chemical processes in the troposphere. This paper describes the first direct measurements of OH in the boundary layer of the tropical Atlantic Ocean. The use of Differential Optical Absorption Spectroscopy provided direct measurements of OH with a calibration uncertainty of 6%. The 1-&sgr; precision of the OH data was in the range of (1--4)¿106 cm-3 because of the exceptional experimental conditions encountered on the ship. On 10 measurement days we collected a total of 483 OH concentration data between 5 ¿N and 40 ¿S. Careful analysis was applied to select data not affected by the ship and its exhaust. The selected data (N=238) exhibit diurnal profiles with maxima around 7¿106 cm-3 for overhead Sun and clean air conditions. On average the measured OH concentrations are 16% higher than corresponding box model calculations based on simultaneously measured trace gas concentrations and photolysis frequencies. The deviation from the 1:1 relation, however, is covered by the combined calibration errors of OH, CO, and the photolysis frequencies. ¿ 2001 American Geophysical Union