A technique was devised for studying non-isothermal changes in properties or compositions. The required data in this technique are the total change in property or composition, the temperature-time behavior, and either some assumptions about the temperature behavior of the system or the expression for the rate of change of a property or the composition. The technique was tested by applying it to two different evaporating systems whose products of evaporation effused through a Knudsen cell orifice. For example, the vapor pressures, P, of silver and boron nitride were determined as a function of temperature, T, both isothermally and non-isothermally. The technique is especially applicable in studying decomposing systems in a mass spectrometer, where the relative concentration of the species being evaporated as a function of time and temperature can be followed. The treatment of data can be adapted to the direct calculation of mass loss, changes in properties of systems, andor changes in composition of various systems, such as ablating systems.