The effect of surface cleanliness on the alkaline hydrolysis of poly(ethylene terephthalate) fibres was investigated using inverse gas chromatography (IBC) in conjunction with mass loss measurements and electron microscopy. The sizing agent was removed from the fibres surface by two methods: soxhlet cleaning in acetone and washing in an aqueous solution of a non-ionic detergent. Alkaline hydrolysis was carried out using two concentrations of aqueous sodium hydroxide, 1% and 10% by mass. The measurement of the specific retention volume of undecane and the heat of adsorption using IGC indicated that the acetone cleaned samples were essentially surface contaminant free, while partial contamination of the surface by the sizing agent remained in the detergent cleaned samples. The presence of sizing agent significantly altered the degree of hydrolysis and the surface topography. The increasing values of the hat of adsorption indicated that significant surface hydrolysis increased the surface crystallinity. (C) 2000 Society of Chemical Industry.