Dehydration of CaCl2-LiCl solution was performed by membrane distillation as a part of thermochemical heat pump. Water molecules in the membrane distillation permeate through the porous membrane, diffuse in the diffusion gap and are condensed finally on the cooling plate. The resistance of water permeation in the membrane distillation is, therefore, considered to be two steps, i.e., permeation through porous membrane and permeation in the diffusion gap. Membrane distillation for 10-40 wt% of CaCl2-LiCl was tested at various temperature differences and different pressure in a diffusion gap (PD). Effect of PD on water permeabilities is discussed. It is suggested that the resistance of water permeation is relatively low in the diffusion gap and that the resistance mainly occurs in water permeation through the porous membrane.