High-performance concrete is sensitive to early-age cracking, mainly due to its rapidly-developing autogenous shrinkage. Autogenous shrinkage and internal relative humidity (RH) decrease are direct consequences of the emptying of capillary pores due to cement hydration in low water-to-cement ratio concretes. To predict early-age cracking, it is desirable to model the evolution of the internal RH, and the accompanying autogenous shrinkage, based on the microstructure evolution. In this paper, the pore size distribution of cement pastes measured by mercury intrusion porosimetry and the chemical shrinkage are used as input data for calculating the internal RH of Portland cement pastes with different water-to-cement ratios (0.30, 0.35 and 0.40). The composition of the extracted pore solution is also taken into account. The measured RH can be predicted with good accuracy (mostly within 1-2% RH). However, a number of sources of error both in the experiments and in the analysis need to be identified and controlled. (c) 2013 Elsevier Ltd. All rights reserved.