A theoretically based predictive model for calculating both liquid and gas phase viscosities has been developed by modifying a statistical mechanics viscosity model based on the square well intermolecular potential. The original theory was corrected to account for the assumptions of only two-body interactions and molecular chaos for velocities and the inadequacy of the square well potential. In addition, the modified model approaches a consistent low-density limit with improved dilute gas temperature dependence. The model parameters are obtained from gas and liquid viscosity data and generalized with corresponding states expressions and group contributions. Gas and liquid viscosities for a wide variety of 174 nonpolar, polar, and hydrogen-bonding compounds are correlated with average deviations of 0.5 % and 1.8 % and predicted with average deviations of 2.2 % and 6.1 %, respectively.