Shear stress versus shear rate and wall shear stress versus slip velocity relations of polymer melts are key material property functions needed in the design of polymer processing equipment. These properties are normally measured using capillary or slit viscometers. The presence of slip greatly complicates the processing of the resulting viscometry data. This paper describes the conversion of the viscometry data of a number of polyvinyl chloride melts through slit dies with different gaps into these two material property functions. The conversion procedure, based on Tikhonov regularization, has the advantage that it is independent of rheological constitutive equation and does not require extrapolation of the experimental data. It also has the ability to cope with the unavoidable noise in the experimental data. Consequently. the property functions thus obtained are likely to be closer to true material properties than those from some of the existing methods. For each of the polyvinyl chloride melts investigated, these proper-ties' functions will be compared with published results and their differences discussed.