A sizing apparatus to coat carbon fibres with aqueous water-dispersible polymeric interphases was developed. The apparatus utilizes a high level of fibre spreading achieved through control of fibre tension and the application of rollers. A statistical process model was developed to relate the mean sizing level to the process independent variables. Pyrolysis in a nitrogen atmosphere was developed as a quantitative sizing level determination technique for the polymer poly(vinyl pyrollidone) (PVP). Scanning electron microscopy demonstrated that the PVP was deposited on to the fibres in a highly uniform manner. The sizing process along with the statistical process model and the characterization techniques allow for the precise development of interphase materials which are tailored for optimal composite performance.