An important potential use of glass fibre is for the reinforcement of Portland cement matrices. However, the high pH of cement, approximately 12.4, degrades ordinary glass rapidly; special glasses containing ZrO2 afford improved durability, but the properties still deteriorate during prolonged wet cure. The literature on the mechanism of deterioration is reviewed; several interpretations have been advanced. New data have been obtained by curing alkali-resistant fibre in cements for up to 90 d at 20 and 55-degrees-C. Two mechanisms of attack predominate; hydroxylation and dissolution of the glass, and notching by Ca(OH)2 crystals growing at the interface between the cement paste and the fibre. The latter is not encountered in aqueous pore fluid simulates, where Ca(OH)2 crystals grow epitaxially on fibres but do not notch them.