Low average Δ18O values and large seasonal variations in Δ18O in snow accumulating on the tropical Quelccaya ice cap in Peru can be quantitatively explained by a balance calculation of water vapor mass and isotopes. Important factors are (1) variations in air mass stability over the Amazon Basin, (2) the surface elevation of Quelccaya, and (3) seasonal changes over Quelccaya. Seasonal changes in evaporation of snow on the surface of the ice cap amplify seasonal Δ18O cycle. Air circulation and air mass stability, rather than temperature, determine the seasonal Δ18O cycle at Quelccaya and, probably, at other low-latitude sites. ¿ American Geophysical Union 1989