A method of predicting fluid phase behavior in a reservoir comprising: a) obtaining at least one sample of a reservoir fluid or mud gas or core and cutting extracts from a reservoir; b) estimating the thermal maturity %RO, of said reservoir fluid: the method being characterized in that it performs via a computer the following steps: c) estimating the API gravity, a condensed gas ratio, CGR and a gas to oil ratio, GOR, of said reservoir fluid using a predetermined correlation with said %RO; d) estimating a hydrocarbon component composition from C1 to C6 and a C7+ fraction, and gas gravity of said reservoir fluid from said API gravity and said CGR and said GOR; e) obtaining critical properties for each of the C1-C6 components from literature in said composition of reservoir fluid; f) splitting said C7+ fraction in to C7 to Cn, where n>7, and estimating mole fractions and critical properties for C7 to Cn components in said composition of said reservoir fluid; g) reducing the number of hydrocarbon components by lumping hydrocarbon components based on any lumping method to form one or more lumped components and zero or more unlumped components; h) building a pseudo equation-of-state, EOS, model using with the compositions, critical properties, Peneloux volume shift parameters and binary interaction parameters for unlumped components and lumped components along with EOS constants for a given EOS model; i) comparing pseudo EOS model calculations with the estimated CGR, GOR, API and gas gravity in steps c-d; j) tuning the pseudo EOS model until the model calculations are within predetermined tolerance limits of said calculations in steps c-d to create a final EOS model, optionally wherein said tolerance limits are 1-10%; and, k) predicting fluid phase behavior and properties with final EOS model. a) obtaining at least one sample of a reservoir fluid or mud gas or core and cutting extracts from a reservoir; b) estimating the thermal maturity %RO, of said reservoir fluid: c) estimating the API gravity, a condensed gas ratio, CGR and a gas to oil ratio, GOR, of said reservoir fluid using a predetermined correlation with said %RO; d) estimating a hydrocarbon component composition from C1 to C6 and a C7+ fraction, and gas gravity of said reservoir fluid from said API gravity and said CGR and said GOR; e) obtaining critical properties for each of the C1-C6 components from literature in said composition of reservoir fluid; f) splitting said C7+ fraction in to C7 to Cn, where n>7, and estimating mole fractions and critical properties for C7 to Cn components in said composition of said reservoir fluid; g) reducing the number of hydrocarbon components by lumping hydrocarbon components based on any lumping method to form one or more lumped components and zero or more unlumped components; h) building a pseudo equation-of-state, EOS, model using with the compositions, critical properties, Peneloux volume shift parameters and binary interaction parameters for unlumped components and lumped components along with EOS constants for a given EOS model; i) comparing pseudo EOS model calculations with the estimated CGR, GOR, API and gas gravity in steps c-d; j) tuning the pseudo EOS model until the model calculations are within predetermined tolerance limits of said calculations in steps c-d to create a final EOS model, optionally wherein said tolerance limits are 1-10%; and, k) predicting fluid phase behavior and properties with final EOS model.