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, S_total, 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-S_total 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.