Since December 1994, activity at Popocat¿petl has evolved through two main phases. The first was characterized by elevated seismic activity, degassing, and emissions of predominantly lithic ash. However, in March 1996 the volcano entered a magmatic phase, which has involved the emplacement of at least eight lava domes within its summit crater. We have used infrared radiance data acquired by the GOES meteorological satellite alongside ground-based geophysical data sets to analyze aspects of this activity. We develop a technique to identify magmatic activity at the volcano. In the absence of a volcanic heat source the radiance emitted from the hottest pixel (Pr) contained within a 10 ¿ 10 pixel GOES subscene centered on Popocat¿petl's summit is well correlated with the average radiance emitted by the remaining pixels (Br) because at any given time all pixels within the target box have similar temperatures, varying as a function of season, cloud cover, and solar irradiance. However, during periods of heightened volcanic activity the radiance emitted by the hottest pixel varies as a function of a time-independent volcanic heat source and changes in Pr and Br become decorrelated, allowing volcanic activity to be identified. Exhalations (intermittent ash emissions) cannot be identified by routine analysis of GOES data because of the low intensity of the associated thermal anomalies. Explosions, however, produce distinctive radiance signatures and can be reliably documented. Two dome growth episodes occurred during our study period, one in November--December 1998 and the other during February 2000. Although the techniques we describe identified the 1998 dome event, the 2000 dome went undetected. This is explained in terms of their contrasting emplacement styles. The 1998 dome was apparent to GOES due to the explosions that disrupted its cool carapace, scattered hot bombs around the summit cone, and exposed its hotter interior to the satellite sensor. In contrast, the February 2000 dome was not apparent because elevated explosive activity did not accompany its emplacement.