The interpretation of the isotopic composition of Andes tropical mountain glaciers is reconsidered using a simple Rayleigh distillation model. It is shown that Andes snowline depression and Huascaran ice core Δ18O are consistent with a 3 ¿C drop in sea surface temperature during the Last Glacial Maximum, accompanied by an increase from 40% at present to 60% during the LGM in the annual mean proportion of atmospheric water lost to continental runoff. Broecker's interpretation of Huascaran Δ18O as a paleo-hygrometer is discussed. ¿ 1999 American Geophysical Union