In a previous publication' (hereafter called I), an approximate cell model was formulated and applied to liquid hydrogen. In this approximate model the potential well of Lennard-Jones and Devonshire2 was approximated by a parabolic well split by a region of constant potential. This approximate cell model appears to preserve the essential features of the Lennard-Jones and Devonshire theory but does not involve the computational difficulties of the exact method of Levelt and Hurst. In this publication the variation of thermodynamic properties among the H2 isotopes is reported. Comparison of Tabulated Properties and Conclusions. -In Table 1 the reduced internal entropy is tabulated for the five H2 isotopes. The reduced thermodynamic functions have been defined in I and the potential parameters (e/k = 37.000K, T = 2.928 A) have been assumed to be the same for all five liquids. The appropriate values of the quantum parameters of these five liquids, obtained from Hirschfelder et al.,4 have also been included in Table 1. The communal entropy correction has been introduced and, therefore, the internal entropy tends to zero at high temperatures and low densities. In Tables 2 and 3 the reduced internal energy and the reduced internal specific heat are tabulated. The internal energy and specific heat are in reasonable agreement with the calculations of Levelt and Hurst.' The agreement of the internal entropy with the calculations of Levelt and Hurst is less satisfactory. As was reported in I, this approximate cell model does not give satisfactory values for volume derivatives. However, it does give reasonable results for the quantum correction. If the quantum correction to the reduced pressure is added to the reduced pressures, calculated by Hirschfelder et al.,4 for the classical cell theory, a good estimate of the critical constants may be obtained. The calculated and critical constants of the five H2 isotopes are given in Table 4. The critical constants for H2, HD and D2 were obtained by Hoge and Lassiter. The experimental critical constants of DT and T2 are given in parentheses, since they are estimates given by Friedman et al. based on an assumed linear variation of the critical properties with X*. For comparison, the calculated and critical constants for a classical liquid have also been included. The calculated