Surveillance of Popocatépetl volcanic plume geochemistry and SO2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993. Volatile traps placed around the summit were collected at near-monthly intervals until the volcano erupted on December 21, 1994. Additional trap samples were obtained in early 1996 before the volcano erupted again, emplacing a small dacite dome in the summit crater. Abundances of volatile constituents (ppm/day of Cl, Stotal, F, CO2, Hg, and As) varied, but most constituents were relatively high in early and late 1994. However, ratios of these constituents to Cl were highest in mid-1994. δ34S-Stotal in trap solutions ranged from 1.5‰ to 6.4‰; lowest values generally occurred during late 1994. δ13C-CO2 of trap solutions were greatly contaminated with atmospheric CO2 and affected by absorption kinetics. When trap data are combined with SO2 flux measurements made through November 1996, Popocatépetl released about 3.9 Mt SO2, 16 Mt CO2, 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H2O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTIR) commonly exceed human recommended exposure limits and may constitute a potential health hazard. Volatile geochemistry combined with petrologic observations and melt-inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at Popocatépetl. Minor assimilation of Cretaceous wall rocks probably occurred in mid-1994.