This paper reports the first simultaneous fast-time resolution measurements of dimethylsulfide (DMS), dimethylsulfoxide (DMSO), and the hydroxyl radical (OH) at marine tropical latitudes. These observations were recorded during the PEM-Tropics B field program on NASA's P-3B aircraft. The observations of DMSO, using a Selected Ion Chemical Ionization Mass Spectrometry (SICIMS) technique, are of particular significance. They have revealed two unique findings: 1) average midday-tropical levels of DMSO are significantly higher than those predicted from current models when constrained by observed DMS and OH levels (e.g., 10 pptv versus 1 to 3 pptv); and 2) DMSO concentration profiles are significantly out-of-phase with model predictions, maximum values being seen under near dark conditions and minimum values being observed at midday. Although no simple explanation has yet been found for these unusual results, the fact that no evidence points to a problem in the measurements suggests that others may exist. Clearly, if the observations are correct, they indicate that at least for tropical upwelling regions the atmospheric sulfur budget may need to be adjusted to accommodate additional sources of DMSO. ¿ 2001 American Geophysical Union