Intrinsic viscosities [η] and Huggins' constants k are generally determined by means of one of the following equations:
It was proven in an earlier paper that eq. (1) is an insufficient approximation of eq. (2) in the usual range ηr = 1.2–2.0. In this paper it is shown that eqs. (2) and (3) can lead to different Staudinger indexes (difference up to 30%). Huggins' constant k can vary from one equation to the other by as much as 300%. The experimental test of theoretical relationships between [η], k, and other molecular constants can lead to errors of the same magnitude. Values of [η] and k in pure and mixed theta solvents are discussed. It is moreover shown that eq. (2) is an inadequate approximation of eq. (3). Therefore eq. (3) is the only one which should be used.