The microstructure, C-S-H composition and the hydrate distribution of the paste phase, close to aggregates and in the bulk, of mature concretes have been studied. Concrete specimens, and non-concrete specimens in which paste is cast against a piece of aggregate, were studied. At a w/c ratio of 0·40 there exists a marked interfacial transition zone around the aggregate particles showing characteristics such as few un reacted cement grains, little C-S-H, much CH, many hollow shell and, presumably, a relatively high capillary porosity. The microstructural gradient towards the aggregate interface decreases if the w/c ratio is reduced or if the mix contains silica fume. AFm and, presumably, AFt appear to exist both in the bulk phase and in the intelfacial transition zone, apparently without any preference for either of the two phases. The chemical composition of the C-S-H seems to be similar in the interfacial transition zone and in the bulk phase.