Atmospheric dimethylsulfoxide (DMSO), an important dimethylsulfide (DMS) oxidation by-product, has been monitored on a daily basis at Amsterdam Island in the southern Indian Ocean during three periods (January--September 1997, January--June 1998, and January--October, 1999). In addition, during an intensive experiment carried out in January 1998, DMSO diurnal variation has been studied in conjunction with parallel measurements of atmospheric DMS. Monthly average atmospheric DMSO mixing ratios range from 0.3 to 5.8 parts per trillion by volume and present a well-distinguished seasonal cycle with a minimum in winter and a maximum in summer similar to that observed for atmospheric DMS simultaneously measured. DMSO presents also a well-distinguished diurnal variation with maximum values around 0900 and minimum during night. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces quite well the diurnal variation of DMSO when assuming a 50% yield from the DMS+OH addition channel <Hynes and Wine, 1996>. However, in absolute terms the simulated DMSO concentrations were about a factor of 2 higher than those measured. Photochemically driven heterogeneous losses of DMSO on aerosol and/or clouds could account for this difference. ¿ 2000 American Geophysical Union