A systematic experimental investigation with the purpose of quantifying the effect of the interactions between non-polar (unmodified) and polar (modified) polypropylene melts and treated fibre surfaces during wetting has been performed. The glass fibres were sized by ammosilane (gamma-APS), gamma-APS/polyurethane film former (gamma-APS/PU) and gamma-APS/polypropylene film former (gamma-APS/PP). Unsized fibres were used for comparison and were also coated with an azidosilane layer. Zeta potential and contact angle measurements were employed to investigate the surface properties of the treated glass fibres. The surface roughness was characterised using both atomic force microscopy and scanning electron microscopy. A method based on axisymmetric drop shape analysis was employed to determine the surface tension of the polymer melt. The wetting of the fibres by the polymer melt was investigated using the Wilhelmy balance technique. The wetting kinetics was different for different fibre surface treatments. The chemically reactive system based on azidosilane showed a better wetting than the other systems. The ammosilane/polypropylene film former (gamma-APS/PP) treated fibre was also characterised by a fast wetting, most probably due to the physical similarity between the polymer and the fibre sizing and, thus, probable compatibility. In the receding case the differences between the reactive systems were smaller than they were for the advancing case. Higher adhesion tension calculated from the wetting measurements correlated well with higher adhesion strength determined from single fibre pull-out tests.