In the present work, a quantitative structure property relationship study is performed to predict gas heat capacity for a structurally wide variety of organic compounds using the genetic function approximation (GFA) and the adaptive neuro-fuzzy inference system (ANFIS) methods. The simple proposed models contain only three descriptors calculated solely from the molecular structure of compounds which are 3D-independent descriptors. The models were validated by an external prediction set. Good results were obtained from both models which get the squared correlation coefficients of 0.996 and 0.997 for GFA and ANFIS, respectively. This study discloses enhanced correlations of the heat capacity of gases with their molecular structures, wherein the influence of the size of molecules is found to predominate.