In this study, the effect of fibre sizing on the modification of basalt fibres in preparation for use with a polypropylene matrix (PP) was investigated. Fibres were coated by the manufacturer with a standard available epoxy (EP) sizing and four experimental PP focused sizings (PPs1-4). Fibre with no sizing was produced to act as a control. The surface topography of sized fibre was analysed by SEM and AFM, indicating that PP sized fibres displayed a more inhomogeneous coating of the fibre. Furthermore, PP sizing resulted in an increase in AFM measured roughness by similar to 360%, translating to a 12.5% increase in surface area, over both unsized and EP fibres. Scratching of the fibre surface revealed, that in general, the coating thickness of PP was similar to 30 nm thicker than EP sizing despite the same application parameters. XPS revealed that the sizing in all cases adhered to the fibre surface with an increase in potential reactive sites present on PP sized fibres. Analysis of fibre surface energy showed that the overall surface energy of fibres remained similar but the use of PP focused sizing resulted in a decrease of the polar component. Overall, this investigation shows that sizing has a significant effect on the fibre's surface: changing its topography and chemistry and hence, has an evident potential for increased mechanical and chemical bonding. This was further confirmed by single fibre fragmentation testing which highlighted that sizings PPs2-4 increase the interfacial shear strength by up to 117% compared to non-sized fibres.