The acentric factor has been calculated for several hundreds of compounds using Edmister’s rule : omega = [(3/7){0/(1 - 0)} log P-c] - 1 where 0 is the reduced normal boiling temperature (= T-B/T-C) and P-C is the critical pressure in atmospheres. On an overall basis, Edmister’s rule represents the acentric factor with an average absolute deviation of 6.2%. The effect of this variation on the calculation of thermodynamic properties is shown to be less than 2%, suggesting the use of Edmister’s rule for the application and development of thermophysical property correlations.