The present study has been carried out in order to know the molecular composition of the beams issued from the electron bombardment of MgO(s) sources and to explain the growth of stoichiometric MgO layers, the feature which is not systematically observed with oxides. The first part deals with mass spectrometric analysis of gaseous species existing in the evaporated molecular beam from a MgO(s) source, and relates these observations with the thermodynamics of MgO Vaporization as well as with earlier free or so-called Langmuir Vaporization rate studies. The necessary congruent vaporization of this stable oxide, MgO, is the key for the evaluation of the partial pressures, either at equilibrium or for free vaporization. The main gaseous species in the beam are Mg(g) and O(g), meanwhile MgO(g) is not detectable and O-2(g) is lower at least by a factor of 6-12 than that is calculated at equilibrium. The evaporation coefficients alpha (ev)(i) are used as usual parameters for evaluation of free vaporization flows obtained at equilibrium. A comparison with earlier Langmuir studies leads us to propose the following set of evaporation coefficients: alpha (ev)(Mg) = alpha (ev)(O) = 1/3, and alpha (ev)(O2) < 10(-2), (ev)(MgO) < 10(-2). The meaning of such values is related to some limiting steps in the processes involved at the surface, i.e., the surface diffusion of the Mg and O adsorbed species and also the low probability that Mg and O react at the surface to produce the MgO and O-2 adsorbed species able to be vaporized.