Abstract In this study, coir fibres were studied for use as reinforcement materials for polymer and cementitious composites. The effect of fibre treatment (i.e. 5 wt.% NaOH solution at 20 °C for 30 min) on microstructure and mechanical properties of coir fibre, coir fibre reinforced epoxy (CFRE) and coir fibre reinforced cementitious (CFRC) composites were investigated. Scanning electronic microscope (SEM) studies were carried out to examine the microstructures of untreated and treated coir fibres, fibre/epoxy and fibre/cement interfaces. Mechanical properties of CFRE were determined by vibration, tensile and flexure tests and mechanical properties of CFRC were determined by compression and four-point bending tests, respectively. The test results show that coir fibre had a much cleaner and rougher fibre surface after the alkali treatment. Compared with the untreated CFRE, treatment improved the tensile and flexural properties of composites, i.e. 17.8% and 16.7% growth in tensile and flexural strength, respectively. However, the treatment also reduced the damping ratio of the CFRE. The increase in tensile and flexural properties and reduction in damping ratio are attributed to the improvement of fibre and epoxy matrix interfacial adhesion due to the treatment, as displayed by SEM micrographs. Compared with the plain concrete, coir fibre improved the compressive strength, flexural strength and toughness effectively. The treatment can further improve these properties of CFRC. SEM studies clearly confirmed that the failure modes of coir fibres in cement matrix are fibre breakage, fibre pull-out and fibre debonding from the cement matrix. The microstructure of coir fibre, CFRE and CFRC were correlated with their mechanical properties.