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Gas heat capacity, C_P, is a fundamental extensive thermodynamic property depending on molecular transitional, vibrational and rotational energy. Empirical four-parameter polynomials approximate the sigmoidal C_P trends for temperatures up to 1500 K and adding parameters extends the range. However, the fitted parameters have no physical significance and diverge beyond their range at high temperature. Here we propose an exponential expression for C_P whose fitted parameters relate to the shape of the C_P versus T curve and to molecular properties: C_P = C_P⁰ + C_P^∞[1 + ln(T)(1 + T_i/T)] exp(−T_i/T). It accounts for more than 99% of the variance with a deviation of ∼1% from 298 K to 6000 K for linear C₁–C₇ hydrocarbons and N₂, H₂O, O₂, C₂H₄, H₂, CO, and CO₂. We also provide an integrated form for enthalpy and an approximation to calculate entropy variations. This model replaces empirical polynomials …