The static method for measuring vapor pressure assumes that the sample is pure and that its temperature is steady and uniform. In practice, the measured pressure may be time dependent because of evaporative cooling after pumping on the sample, transpiration of the sample in a temperature gradient, or diffusion of an impurity out of the sample. An impurity cannot be avoided if the sample is decomposing. This article identifies and quantifies various causes of time dependence, and it includes an analysis that can obtain the vapor pressure from the time-dependent pressure of a decomposing sample. The analysis was applied to measurements of TEMAH (tetrakisethylmethylaminohafnium), whose decomposition continuously generated a volatile impurity. The corrected vapor pressures obtained for three TEMAH samples at 39 degrees C agreed to within +/- 1 %, even though the partial pressure of the impurity was as much as 8 times larger.