Polytetrafluoroethylene (PTFE) microfiltration membranes are commonly used in Membrane Distillation (MD) systems, and parameters such as the pore size and porosity have significant influence on their performance. The operating temperature of a membrane distillation unit is typically 60-80˚C, and while PTFE is considered to be thermally stable it does expand with increasing temperature. When dealing with a porous microstructure this expansion becomes significant. It was found that increasing the membrane temperature resulted in an expansion of the fibrous PTFE material and subsequently an increase in pore size. The membrane structure was observed over a period of 80 minutes, this time was deemed necessary given that PTFE has low thermal conductivity and therefore would heat up slowly. Pore size increased by 32% in the first 60 minutes, when the sample was heated to 80˚C. A lumped system analysis of the heat transfer inside the SEM chamber was used to determine a heat transfer coefficient of 0.72 W/m2K. The temperature dependence of pore size will result in fluctuations in performance when the membrane is used intermittently and therefore heated and cooled periodically.