The results of the investigation on durability of vegetable fibre-mortar composites subjected to three different environmental conditions; immersed in water, exposed to the open air weathering and controlled cycles of wetting and drying are presented in this paper. The performance of the composites is discussed using results of flexural tests carried out after 28 days, which are considered as reference data, and after 6 and 12 months of ageing. To observe the modification of the composite material micrographs of the fibres obtained from aged specimens have been analysed. It was found that composites manufactured with sisal fibres and Portland cement matrix presented a significant reduction in toughness after 6 months of weathering or submitted to cycles of wetting and drying due to fibre mineralisation by calcium hydroxide. To improve the durability of the composites, two types of treatments were studied: carbonation of the cementitious matrix and immersion of the fibres in slurry silica fume prior to being incorporated in the Portland cement matrix. The results obtained after 12 months of ageing showed that these treatments prevented the embrittlement of the composites.