In this study a temperature-dependent function of the attractive term, called the alpha function, and the critical compressibility factor (zeta(c)) of the ER (Esmaeilzadeh-Roshanfekr) equation of state have been modified to improve the performance of equation of state. The ER EOS is a new three-parameter equation of state that was developed in 2006 with special attention to application for reservoir fluids. Using these modifications, saturated liquid density, saturated vapor density and vapor pressure of pure components are calculated more accurate than the ER equation of state. The average absolute deviations of the predicted saturated liquid density, saturated vapor density and vapor pressure of pure components with the modified ER (mER) are 0.95%, 1.17% and 1.20%, respectively. Also, the mER, the ER and the Peng-Robinson (PR) equation of state are used to predict dew point pressure for six gas condensate mixtures and bubble point pressure for five LNG mixtures. In order to have an unbiased comparison between these equations of state, van der Waals mixing rules are used without using any adjustable parameters (k(ij) = 0). The results show that the mER EOS predicts the dew point pressure and bubble point pressure with best accuracy among the other EoSs. (c) 2008 Elsevier B.V. All rights reserved.