The comparatively novel technique of nanoindentation has been used to provide unique information on the hardness and modulus of artificially weathered window-grade unplasticised polyvinyl chloride (UPVC) at the sub-micron level. Indentations, in plan, were made to depths ranging from about 140 nm to 5.5 mu m while indentations in section were made at distances ranging from about 2.5 mu m to 1.0 mm from the top surface. The technique was applied also to Darvic(R) UPVC which, by virtue of its mo re homogeneous microstructure, was used as a reference material for assessing the effect on properties of the detailed loading procedure. The sensitivity of the hardness to loading rate and to hold-time at maximum load was investigated. Hold-time was the key variable and an optimum period of 200 s was recommended. The hardness of the artificially weathered window-grade UPVC varied with exposure period and with depth. At long exposure times, the hardness of the top 147 nm was very much greater than the underlying material, which had softened with exposure. This very-near surface hardening, which is attributed to cross-linking, would not be detected by conventional techniques. After extended exposure, evidence of recovery of properties was observed, consistent with the mechanism of surface layer removal by erosion. The modulus was less affected by weathering than hardness with a modest decrease being observed only at very prolonged exposures. Measurements of hardness in section were not consistent with measurements in plan suggesting that the sectioning and polishing process was inducing changes in properties in the near-surface region. This problem may restrict the application of the technique for detailed depth profiling close to the weathered surface, at least for some materials. Use of the nanoindentation technique for detecting the early onset of degradation is feasible but more work is required to understand the depth dependence of degradation in relation to hardness and to isolate this effect from local material variability.