Addition of tert-butylisocyanide or 2,6-dimethylphenylisocyanide to a solution of trialkylaluminum or trialkylgallium results in formation of complexes R3M center dot C (NBu)-Bu-t (M = Al, R = Me (1), Et (2), Bu-t (3), Bu-t (4); M = Ga, R = Bu-t (9)) or R3M center dot C N(2,6-Me2C6H3) (M = Al, R = Me (5), Et (6), Bu-t (7), Bu-t (8); M = Ga, R = Bu-t (10)), respectively. Complexes 1, 4, 5, and 8-10 are isolated as solids, whereas the triethylaluminum and triisobutylaluminum adducts 2, 3, 6, and 7 are viscous oils. Complexes 1-10 were characterized by NMR (H-1, C-13) and IR spectroscopies, and the molecular structures of 4, 5, and 8-10 were also determined by X-ray crystallography. The frequency of the C N stretch of the isocyanide increased by 58-91 cm(-1) upon complexation, consistent with coordination of the isocyanide as a sigma donor. Enthalpies of complex formation for 1-10 were determined by isothermal titration calorimetry. Enthalpy data suggest the following order of decreasing Lewis acidity: (Bu3Al)-Bu-t >> (Bu3Al)-Bu-t >= Me3Al approximate to Et3Al >> (Bu3Ga)-Bu-t. In the absence of oxygen and protic reagents, the reported complexes do not undergo insertion or elimination reactions upon heating their benzene-d(6) solutions to 80 degrees C.