Mercury intrusion porosimetry (Mill) test is one of the techniques that have been widely used for analyzing the pore size distribution of hardened cement paste (hcp) and also for the determination of the critical pore width. This study presents the test results of the MIP experiments obtained for three different hcp specimens with the water-cement ratios of 0.26, 0.34, and 0.42 which had been cured for 7, 28, and 365 days under water. Thus, the effects of water-cement ratio and curing time on the critical pore width of hcp were investigated. Test results have shown that, within the limits of the work, and in case of complete hydration, the critical pore width of the hcp seems to be independent of water-cement ratio and is of the order of 25 nm. This value can be considered as the critical pore width of the portland cement: gel. (C) 2008 Elsevier Ltd. All rights reserved.