We have examined the effect of varying opacities on envelope structure with the aid of Paczynski's public domain stellar modelling programs. For this, we prepared new opacity tables from the data of Kurucz , using Lagrange interpolation to obtain the tabular values. We compare the results of these Kurucz opacities with similar tabulations from Huebner et al. , Iglesias and Rogers , Rogers and Iglesias  and Iglesias and Rogers . We have checked calculations for the same ranges of stars considered originally by Schwarzschild , and compared our findings, using newer opacity data, with those of other sources. We consider how such calculations relate to high accuracy observational data, with the well-observed planetary eclipsing system V 376 Peg (HD 209458), providing a guideline towards data of similar accuracy in the near future. Current accuracies on absolute radii and masses derivable from eclipsing spectroscopic binaries are conservatively estimated at similar to 1%. The effects of revised opacity calculations on the radii of stars of intermediate mass are several times greater than this (5-10% for constant values of other parameters), so that eclipsing binary data should have good potential for independent tests of opacity theory across a wide range of stellar types.