Studies on the equilibrium between austenitic iron and nitrogen under various conditions of the maximum pressure of 920 kg/cm^2 and the temperature range from 950°to 1300℃ were carried out by using a high temperature and high pressure equipment. It was shown that the concentration of nitrogen in austenite deviated from Sieverts\u27 law with increasing pressure. The casue for such discrepancy was considered thermodynamically and statistically. That is, the experimental result can reasonably be explained from the geometrical consideration that the chemical potential of a nitrogen atom in austenite, in case of a high nitrogen content deviates remarkably from that in an idial random interstitial solid solution, since each intestitial atom added excludes other interstitial atoms from the seven adjacent sites. Thus, the activity of nitrogen in austenitic iron can be expressed by the following equations : log a_N=log N_N/1-9N_N+ 3.34-427/T for the standard state of nitrogen gas 1 atm pressure, a_N=f^_ log a_N=log N_N/1-9N_N for the standard state of infinitely dilute solid solution
