A recently developed image-intensity-matching technique has been used to analyse images of cement paste which were dried in an environmental scanning electron microscope. Shrinkage that occurs during changes in relative humidity is reported, together with some of the influences of water-to-cement ratio, temperature and age. Results from microstructurally based models are compared with experimental results. The best fit of models to experiment is achieved if calcium silicate hydrate (C-S-H) is divided into two types : high density C-S-H, which does not shrink, and low density C-S-H, which does shrink. Approximate values of unrestrained shrinkage of the low density C-S-H are attained as a function of relative humidity.