Knowing accurate saturated vapor pressures of explosives at ambient conditions is imperative to provide realistic boundaries on available vapor for ultra-trace detection. In quantifying vapor content emanating from low-volatility explosives, we observed discrepancies between the quantity of explosive expected based on literature vapor pressure values and the amount detected near ambient temperatures. Most vapor pressure measurements for low-volatility explosives, such as RDX (1,3,5-trinitro-1,3,5-triazinane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane), have been made at temperatures far exceeding 25 degrees C and linear extrapolation of these higher temperature trends appears to underestimate vapor pressures near room temperature. Our goal was to measure vapor pressures as a function of temperature closer to ambient conditions. We used saturated RDX and HMX vapor sources at controlled temperatures to produce vapors that were then collected and analyzed via atmospheric flow tube-mass spectrometry (AFT-MS). The parts-per-quadrillion (ppq(v)) sensitivity of AFT-MS enabled measurement of RDX vapor pressures at temperatures as low as 7 degrees C and HMX vapor pressures at temperatures as low as 40 degrees C for the first time. Furthermore, these vapor pressures were corroborated with analysis of vapor generated by nebulizing low concentration solutions of RDX and HMX. We report updated vapor pressure values for both RDX and HMX. Based on our measurements, the vapor pressure of RDX at 25 degrees C is 3 +/- 1 x 10(-11) atm (i.e., 30 parts per trillion by volume, ppt(v)), the vapor pressure of HMX is 1.0 +/- 0.6 x 10(-14) atm (10 ppq(v)) at 40 degrees C and, with extrapolation, HMX has a vapor pressure of 1.0 +/- 0.6 x 10(-15) atm (1.0 ppq(v)) at 25 degrees C.