Objective Small chemical hand and foot warmers are used by many winter sport athletes and other outdoor enthusiasts. The purpose of this study was to characterize the thermal behaviors of 14 commercially available hand and foot warmers. Methods Both types of warmers were instrumented with a fast-responding thermistor. One of each pair of warmers was placed in a boot or glove. Temperature was recorded in a data logger for a period of hours to more than a day until the heat output of the devices ceased and returned to room temperature. Thermal behaviors were characterized and graphed, including peak temperature, time to peak temperature, and duration of temperature greater than 30°C. Reliability was determined by comparing thermal behavior of 2 hand or foot warmers of the same type and from the same manufacturer. Comparisons were also made between package claims and measured behaviors. Results Measured outcomes were most noteworthy in their variability within and between manufacturers and types of warmers. A strong correlation was found between the masses of the warmers and their duration of heat production. Conclusions Although there is a paucity of research information on these types of warmers, this experiment showed that some of the devices exceeded packaging claims while others fell short; that the thermal behavior over time of the devices was variable; and that there appears to be a simple but strong relationship between the mass of the devices and the duration of their heat production.