Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below I mu m) caused that the air permeability was reduced from 1.365 to 0.863 dm(3)/m(2) s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO3 deposit formed during distillation of tap water. This deposit was removed every 30-70 h by rinsing the modules with a 2-5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores. (c) 2006 Elsevier B.V. All rights reserved.