A vacuum membrane distillation (VMD) method was employed for the direct determination of configurational number r epsilon/tau for porous membranes used in the MD process. For the same purpose, the transport mode was investigated experimentally. Results showed that mass transfer of the MD process is predominated by Knudsen diffusion in our experimental system. Although greatly discrepant from the predicted value based on assumption, the directly obtained r epsilon/tau value correlates well with flux, pressure and temperature.