Additivity methods for heats of vaporization of organic liquids have been developed using both bond and group additivity parameters. For the bond level parameters, 429 compounds could be fitted using 36 parameters, with a weighted standard deviation of 0.32 kcal/mol, and only 23 deviations of more than 1 kcal/mol. For the group level parameters, 388 compounds could be fitted using 57 parameters, with a weighted standard deviation of 0.19 kcal/mol, and only 7 deviations of more than 1 kcal/mol. The problem of correlation among parameters is discussed and the significant terms in the correlation matrix are given. The new parameters are compared to previous parameterization schemes.