This paper presents the results of an experimental program designed to assess the durability of sisal and coconut fibres exposed to alkaline solutions of calcium and sodium hydroxide. In addition, the durability and microstructure of the cement mortar composites reinforced with these fibres aged under tap water, exposed to controlled cycles of wetting and drying, as well as to the open air weathering have been studied. The possibility of biological attack of fibres was investigated by conditioning them in tap water for 420 days. It was found that sisal and coconut fibres kept in a calcium hydroxide solution of pH 12 completely lost their flexibility and strength after 300 days. The composites manufactured with short sisal or short coconut fibres and ordinary Portland cement `OPC' matrix presented a significant reduction in toughness after six months of exposure to the open air weathering or after being submitted to cycles of wetting and drying. The embrittlement of the composites can be mainly associated with the mineralization of the fibres due to the migration of hydration products, especially calcium hydroxide, to the fibre lumen, walls and voids.