The interest of using natural fibres in composite materials has greatly increased over the past decades thanks to their good mechanical properties in combination with environment-friendly characteristics. Among natural fibres, coir fibres are not very strong and stiff, but have high strain to failure which may increase toughness of some brittle matrices when they are used in composites. Vietnamese coir fibres are studied and modified for use in composite materials. In order to achieve a good performance of composites, it is important to understand the quality of the interfacial adhesion between fibres and matrices. Based on fibre-matrix compatibility in terms of matching surface energies, either fibre treatment or matrix modification is chosen to improve the compatibility. In this study, the interface of unidirectional (UD) composites of untreated and 5% alkali treated coir fibres in both polypropylene and epoxy matrices are investigated using wetting analysis and interfacial mechanical tests. Wetting measurements of the fibres and the matrices are carried out to obtain their static contact angles in various liquids, which are used to estimate the surface energies comprising of different components. The work of adhesion is calculated for each composite system, accordingly. Besides, fibre surface chemistry is also examined by X-ray photoelectron spectroscopy (XPS) to have more information about functional groups at the fibre surface, which assist a deeper understanding of the interactions at the composite interfaces. The result shows a carbon rich surface consisting of waxes and a low proportion of lignin and fatty substances, which reflects well the hydrophobic properties of coir surface found by wetting measurement. To determine the quality of composite interfaces, transverse three point bending tests are performed on UD composites to measure interfacial strength. From the wetting analysis, the results show that the work of (physical) adhesion of both untreated and treated fibres with cured epoxy is higher than in case of polypropylene. When comparing untreated and treated fibres, the surface energy of treated fibres is increased, leading to a higher work of adhesion of their composites than in case of untreated fibre composites. The result of the surface energy analysis is quite consistent with the interfacial strength of the composites (even if chemical adhesion mechanisms have not been considered so far). Mechanical properties of the UD composites are assessed by flexural tests in fibre direction and unnotched Izod impact tests. In agreement with the interface evaluation, higher flexural strength and stiffness are found in treated fibre composites, probably thanks to the better interfacial wetting and adhesion. The impact strength of coir polypropylene composites is not significantly different from that of neat polypropylene, while coir fibres can improve the toughness of the epoxy by minimum a factor of three when the impact strength is considered as toughness indicator.