The results of the theory of intrinsic viscosity based on the model of a worm-like chain have been used in analyses of the [η]-M correlation under theta conditions, and of the Stockmayer-Fixman plot for polymers with flexible chains at molecular weights M < 105. Their viscosity behaviour appears to be influenced by the chain thickness d. An analysis of the available data allowed us to obtain reasonable d values for several polymers: polyethylene; poly(ethylene oxide); polystyrene; poly(methyl methacrylate), and poly(vinyl acetate). The effect of solvent on d was ascertained. The analysis shows that the reliability of the results obtained by the Stockmayer-Fixman method is the higher, the larger the proportion of data used which reside in the region M > 105. © 1979.