In many elaboration furnaces, ceramics are used with other materials in vacuum atmospheres. If temperatures are sufficiently high, vaporizations occur. Depending on the thermodynamic stability of the investigated oxide, this vaporization can be congruent or not. In this last case, chemical reactions with the ceramic can take place which can lead to the destruction of it. One way to study these processes is the use of the High Temperature Mass Spectrometry (HTMS). This technique allows determining the composition of the gas phase and the partial pressures of the different gaseous species. By combining the spectrometer to a multiple Knudsen cell furnace, it is also possible to determine activities by direct comparison of the partial pressures of each gaseous species in equilibrium with a mixture and with pure components or compounds in the same experiment. Another recent development of the above technique is to characterize non equilibrium states by determining evaporation and condensation coefficients. These coefficients describe the difference between the real vaporization state of the system and the equilibrium state. To perform such determinations it is necessary to vary the shape of the Knudsen cell orifices in order to change the net evaporation process at the surface of the samples. Theses coefficients can be used to obtain better modeling in processing.