Abstract: A parallel-plate diffusion gas dehumidifier has a treatment zone having at least one water-permeable membrane. The gas dehumidifier includes an untreated gas inlet, a treatment zone bounded by water-permeable membranes, a support structure for the membranes, access to a source of vacuum, and a dehumidified gas outlet. The cross section of the treatment zone may be provided in various shapes, for example, rectangular. The gas dehumidifier inlet and outlet include flow transitions that minimize the obstruction of particles passing through the dehumidifier. The dehumidifier may be used in particle sampling systems to dehumidify the sample gas prior to introducing the sample gas to a mass measuring device and mass flow controller. Methods of operating the gas dehumidifier and the particle sampling system are also provided.