Using a GC headspace measurement technique, the vapor pressure of TATP was determined over the temperature range 12 to 60 degrees C. As a check on the experimental method, TNT vapor pressure was likewise computed. Values for TNT are in excellent agreement with previous published ones. For TATP the vapor pressure was found to be similar to 7 Pa at ambient conditions. This value translates to a factor of 10(4) more molecules of TATP in air than TNT at room temperature. The dependence of TATP vapor pressure on temperature can be described by the equation log(10)P(Pa) = 19.791 - 5708/T(K). Its heat of sublimation has been calculated as 1.09 kJ/mol.