We present an explicit representation of molar enthalpy for gaseous substances based on the improved Rosen-Morse oscillator to describe the internal vibration of a molecule. We calculate molar enthalpy values of gaseous phosphorus dimer in terms of experimental values of three molecular constants, and obtain excellent agreement between the calculated values and experimental data in a temperature range of 298-6000 K. (C) 2018 Elsevier Ltd. All rights reserved.