Liquid adducts of the general formula AsX3L (X=Cl, Br; L=N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), tetramethylurea (TMU)] were synthesized and characterized by elemental analysis, IR spectroscopy and mass spectra. From enthalpies of dissolution (Delta(i)H(m)(0)) in 1,2-dichloroethane at 298.15 K and the appropriate thermochemical cycle, standard molar enthalpies have been determined for Lewis acid-base reaction (Delta(r)H(m)(0)), formation (Delta(f)H(m)(0)), decomposition (Delta(D)H(m)(0) or Delta(M)H(m)(0)), Lewis acid-base reaction in gas phase (Delta(f)H(m)(0)) and the mean arsenic-oxygen bond enthalpies, [D] (As-O). These thermochemical parameters give the acidity AsCl3>AsBr3 and basicity TMU>DMA>DMF.