The durability of alkali-resistant glass fibre in cement matrices with and without silica fume was investigated. Several attack modes such as hydroxylation, mass dissolution and notching by calcium hydroxide crystals were distinguished. The effect of silica fume addition was found to be slight; it greatly reduced the calcium hydroxide content of the cement matrix and inhibited notching attack but it did not reduce the internal pH sufficiently to inhibit hydroxylation and mass dissolution. The flexural strength of cement pastes at 20°C with and without silica fume initially increased during the first month but thereafter started to decrease and eventually levelled off at longer ages. The addition of silica fume gave only a marginal improvement to the elastic properties of composites at 20°C. At 55°C, the flexural strengths of both formulations were observed to decrease very rapidly, approaching the flexural strength of the unreinforced matrix.