Properties of petroleum fractions of unknown composition, the so-called undefined petroleum fractions, are estimated using an approach based on group contribution methods. Using liquid density and molecular weight as experimental data and classical thermodynamics as framework, functional groups were assigned to each undefined fraction by minimizing its free energy. Thus, methods requiring molecular structures were used directly in phase equilibrium simulations of petroleum fluids. The proposed procedure also allows the critical properties of such undefined fractions to be calculated employing no specific correlations. The obtained results show reasonable accuracy concerning phase equilibrium experiments.