Major, trace and rare earth element compositions and 87Sr/ 86Sr ratios of waters from the thermal springs and crater lake at El Chichón volcano suggest the existence of two distinct thermal aquifers (Aq. 1 and Aq. 2). Na/Kand K/Mg ratios indicate that the Aq. 2 composition represents a deep mature hydrothermal fluid. The Ca/Sr ratios are unusually low for Aq. 2 (∼17) compared to Aq. 1 (∼130) and not obviously related to Ca/Sr in the wall rocks. On the other hand, based on the similarity of 87Sr/ 86Sr from waters and wall rocks, we suggest that Aq. 1 is shallow and composed of volcanic rocks, while Aq. 2 is deeper and composed of sedimentary rocks. The REEconcentrations are strongly pH-dependent and for acidic waters match the pattern of El Chichón volcanic rocks.