The surface tension of polymeric materials commonly varies in the range 20-50 mJ/m2. Knowing this fact, the contact angles of 31 polymeric materials were varied with four probe liquids (i.e. diiodomethane, formamide, glycerine and water) in 1 mJ/m2 increment in the mentioned range by the aid of the equation of state (i.e. Neumann's equation). All the 11 possible 2, 3 and 4 combinations of the liquid mixtures were used. The obtained contact angles were then employed to calculate the corresponding surface tensions using different approaches such as the Owens/Wendt, the harmonic mean, the van Oss et al. and the combined mean relationships. Additionally, the differential forms of the mentioned relationships were used to rank the precision of each approach. The results illustrated that the relative differences between the predicted surface tensions obtained from each approach as compared to the equation of state were strongly dependant on the liquid system used. The predicted surface tensions calculated by the Owens/Wendt, the harmonic mean, the van Oss et al. and the combined mean relationships were generally lower than the corresponding values as calculated by the equation of state for low values of surface tension. This behavior was inversed for the upper range of surface tension. Furthermore, the precision ranking of these relationships were in the order of the equation of state> the harmonic mean> the combined mean> the Owens/Wendt> the van Oss et al.