Changes of microstructure and properties in the interfacial zone of glass fibre reinforced cement (GRC) under the effect of ageing were investigated. A novel technique based on a microindentation apparatus was developed and successfully used to carry out microstrength testing in the interfacial area and push-in tests on selected individual fibres within a strand. By continuously monitoring load vs. displacement, the new technique allowed the microstrength to be measured in small, porous areas of the fibre-matrix interfacial zone, and particularly within the glass fibre strand/bundle. The results showed that the embrittlement of aged GRC was closely associated with a substantial increase of the microstrength values within the fibre bundle during the ageing process. It was also revealed that a wide range of bond properties existed within the fibre strand. The resistance to fibre sliding was much greater at the outer filaments than at the inner central filaments of the fibre strand/ bundle.