The new adducts MI 3·L (M = Sb or Bi; L = N,N -dimethylthioacetamide (dmta) or N,N-dimethylthioformamide (dmtf), were isolated, characterized, and studied through reaction-solution calorimetry at the temperature 298.15 K: MI 3(cr)+L(l, cr) = MI 3·L(cr); Δ rH o m(kJ·mol -1) : SbI 3·dmta, -10.58±0.14; SbI 3·dmtf, -14.90±0.14; BiI 3·dmta, -9.96±0.29; and BiI 3·dmtf, -15.19±0.41. Using Δ rH o m and the standard molar enthalpies of formation of ligands and triiodides, the standard molar enthalpies of formation of the crystalline thioamide adducts were calculated; their values for this sequence are: (-177.6, -131.3, -226.6, and -181.2) kJ·mol -1. The mean metal-sulphur bond enthalpies for the same sequence: (112, 117, 144, and 150) kJ·mol -1, were also calculated. © 1993 Academic Press. All rights reserved.251113191324