A modified gas saturation method was used to directly determine the solid vapor pressure of ammonium perfluorooctanoate (APFO). Measurements were made over the temperature range from (45 to 60) degrees C. Pressures ranged from (0.04 +/- 0.007) Pa to (0.17 +/- 0.028) Pa. A relationship was developed for determining vapor pressure as a function of temperature for conditions near the test range, and an environmentally relevant vapor pressure of (0.003 +/- 0.0005) Pa was calculated at 25 degrees C. This ambient vapor pressure for the ammonium salt of perfluorooctanoate (PFO) is 3 orders of magnitude lower than the ambient vapor pressure of the acid form, perfluorooctanoic acid (PFOA), which is approximately 4.1 Pa at 25 degrees C. This finding underscores the importance of knowing what compound is present in the environment and using the appropriate properties to be able to predict environmental fate and transport behavior. The enthalpy of sublimation was calculated as 90.9 kJ.mol(-1). It is hypothesized that APFO may undergo a multistage sublimation process.