An attempt is made at a quantitative interpretation of the low evaporation coefficients alpha (10(-7)-10(-1)) observed under thermal decomposition of 12 different compounds (AgNO3, BaSO4, BaCO3, BN, Be3N2, Mg3N2, AlN, GaN, ZnO, CdS, CdSe and NH4Cl) in vacuo under the conditions of free-surface evaporation after Langmuir. It is suggested that the main reason for the low evaporation coefficients lies in the difference between the true scheme of thermal decomposition of a given compound and in assuming its direct decomposition to the products corresponding to thermodynamic equilibrium. These differences consist, firstly, in a primary gasification of all decomposition products, including low-volatile components (metals and metal oxides) and, secondly, in a partial or total evolution of nitrogen, oxygen, sulfur, selenium and hydrogen chloride in the form of free atoms or H + Cl mixture rather than of molecules. The interpretation of the evaporation coefficients a for elemental red phosphorus, arsenic and antimony is based on the assumption that the sublimation of these substances occurs, accordingly, through a primary formation of P-2 molecules, or of As-2 + As-4 and Sb-2 + Sb-4 mixtures, rather than by direct formation of the P4, As-4, or Sb-4 molecules corresponding to thermodynamic equilibrium, as is generally accepted. The proposed mechanisms of decomposition or sublimation are validated by a comparison of the experimental and calculated activation energies.