VP/VS relations are key to the determination of lithology from seismic or sonic log data, as well as for direct seismic identification of pore fluids using, for example, amplitude variation with offset (AVO) analysis. While there are a variety of published VP/VS relations and VS prediction techniques, most reduce to two simple elements: (1) for each lithology, establish empirical relations among VP, VS, and porosity for one reference fluid—usually water—and (2) use Gassmann's (1951) relations to map these to other pore fluid states. In this short note we point out similarities between the critical porosity models of Nur (1992) and Krief et al. (1990) for predicting VP/VS. Both are useful, primarily because they incorporate the above two robust elements. Although derived from different directions, they are nearly identical.