Three of the most important equations for evaluating the activation energy of a chemical reaction are discussed in the present paper : a modification of the Kissinger equation, a graphic method and a derivative method. They are based on thermal analysis measurements under non-isothermal conditions. These mathematical approaches were applied to the : study of the crystallization of quaternary glassy systems RO-MgO-Al2O3-SiO2 (where R is Ca, Sr, Pa, Zn), which separate from two to four crystalline phases. The nucleation mechanism was also determined by equations applied to DTA curves. Although the activation energy values obtained are similar to those given in the literature,