Some 550 years ago (1320--1433 A.D.), a powerful Plinian eruption at El Chich¿n Volcano in southern Mexico produced a widespread pumice fall deposit. We subdivided the deposit into three parts on the basis of structural and textural characteristics, pumice lithology and density, granulometry, and petrologic-geochemical attributes. The deposit covers an area of 1500 km2 within the 1-cm isopach and has a minimum estimated bulk volume of 2.8 km3 (1.1 km3 dense rock equivalent (DRE)); its eruptive column reached an altitude of ~31 km. Consideration of field evidence, the presence and nature of mafic enclaves, and chemical data strongly suggest that the 550 year B.P. eruption is linked with the intrusion of a high-temperature basaltic magma into preexisting but stagnated trachyandesitic magma beneath El Chich¿n. Thorough mixing of the two magmas produced a compositionally uniform hybrid trachyandesite magma (average SiO2 55.3 wt %), which subsequently underwent crystal growth and gas exsolution, ultimately overpressurizing the zoned magmatic system to erupt explosively. On the basis of El Chich¿n's known eruptive history, the intrusion-mixing event occurred sometime after the 900 year B.P. eruption. The hybrid magma had a preeruption temperature of 820--830¿C and was water undersaturated (5--6 wt % H2O) at pressures of ~2--2.5 kbar.