Isobaric heal capacities C-p have been measured, from 323 to 423 K at pressures from 5 to 25 MPa, by direct calorimetry for different gaseous mixtures containing between 83 and 99% of methane and usually encountered in natural gas. The experimental technique has been calibrated and checked using pure gases such as nitrogen, helium, methane, carbon dioxide and argon. Experimental results were used to evaluate the performance of five equations of state in the case of gaseous mixtures and of two equations of state for carbon dioxide and argon, respectively. The equation of state AGA 8 specially developed to represent pVT-properties of natural gas gives the best results, yielding C-p values which agree within +/- 2% with experimental data.