On account of the high accuracy with which spectroscopic data are obtained, thermodynamic parameters estimated from these data are more reliable than those obtained by direct thermal measurements. Spectroscopic data of aluminum based diatomic gas molecules (AlO, AlH, AlD, AlF, AlC) and partition function theory have been used to evaluate the thermodynamical parameters in the temperature range 100 -3000 0 K. Equilibrium constants for astrophysically important chemical process have been calculated using partition function theory. It is noticed that for AlC the heat capacity decreases exponentially after the critical temperature 1200 0 K.