A colorimeter equipped with a gas booster in conjunction with a PVT cell was used to measure the heat capacity of natural gas with different amounts of impurities. Based on new experimental and literature data, a general investigation of the isobaric specific heat capacity was carried out using the Jarrahian-Heidaryan equation of state (J-H-EOS). A model was obtained that is valid in wide ranges of pressures (0.1-40 MPa) and temperatures (250-414K). The arithmetic average of the model's absolute error is acceptable in engineering calculations and has superiority over other methods in its class. (C) 2014 Elsevier B.V. All rights reserved.