The adducts MI3.tmtu (M = Sb, Bi; tmtu = tetramethylthiourea) were prepared, characterized and studied through reaction-solution calorimetry at 298.15 K. The molar standard enthalpy DELTA(R)H(m)empty set of the reaction MI3(cr)+tmtu(cr)=MI3.tmtu(cr) gave -19.84 +/- 0.46 and -13.01 +/- 0.17 kJ mol-1 for Sb and Bi, respectively. From DELTA(r)H(m)empty set values the standard molar enthalpies of formation of the crystalline Sb and Bi adducts were calculated as -158.5 and -201.3 kJ mol-1, respectively. The mean element-sulphur bond enthalpies were also calculated as 122 and 147 kJ mol-1 for Sb-S and Bi-S bonds, respectively. The acidity order AsI3 > SbI3 > BiI3 was established.2061318