The Lower and Upper Toluca Pumice Formations are dacitic airfall deposits that were erupted about 24,500 yr BP and 11,600 yr BP respectively. The Lower Toluca Pumice covers about 400 km², with a volume of 0.33 km³ (dense rock equivalent 0.16 km³) within the 10 cm isopach, and has a northeast-trending dispersal fan. The Upper Toluca Pumice covers more than 2,000 km², has a volume of 2.3 km³ within the 40 cm isopach, and an estimated total volume of 3.5 km³ (dense rock equivalent 1.54 km³). Its symmetrical dispersal fan trends N 65° E. The Inman parameters, median diameter (Md_φ) and deviation (σ_φ), together with the fragmentation indices, derived from granulometric studies of samples from 23 pumice sections, show that both eruptions were of Pliniantype. The particle-size distribution, compared with a simple mathematical model of a Plinian eruption, provides an approximate indication of the eruptive conditions during the deposition of the coarsest part of the Lower Member of the Upper Toluca Pumice. Computations give a muzzle velocity of 500 m/sec from a circular vent 260 m in diameter, a mean horizontal windspeed of 16.2 m/sec and an eruptive cloud 40 km high, with a maximum rate of release of kinetic energy of 6 × 10¹⁹ erg/sec. The younger eruption released about ten times more kinetic energy than the older.