An empirical equation: Delta(D)H(m)(O) = t(i)/2.2(2-n) is obtained and tested for 102 adducts (mainly adducts with zinc group halides). In the equation, t(i) is the Kelvin temperature of the beginning of the thermal decomposition of the adduct, (obtained by thermogravimetry), and n is the number of ligands. For 1/3 of the tested adducts the difference between experimental and calculated values was less than 5%. For about 1/3 of the adducts that difference exceeds 15%.214437440