The process of decomposition of supersaturated tetragonal martensite during the first stage of tempering is investigated by considering the characteristics of motion of a carbon atom from one interstitial position to another within the martensite matrix. It is evident that not all the carbon atoms take part simultaneously in the process of precipitation, some remain in solution for a longer time, others shorter, some mobile, others stationary. By assuming that each transition from one state of motion to another has a definite probability, which is independant of time and carbon concentration but dependant on temperature and the nature of crystalline matrix, it can be shown that an isothermal decomposition curve can be expressed as a series with exponential terms, the coefficients of which are related to the distribution of carbon atoms in different states of motion. The agreement between this expression and the existing experimental data is satisfactory. The general tendancy of the variation of the coefficients with temperature is just what should be expected.