The complexes AsX3.nL (where L = pyridine-N-oxide (pyNO), alpha, beta, or gamma-picoline-N-oxide (alpha, beta-, or gamma-piCONO), n = 1 or 2 for X = Cl, n = 1, 3/2 or 2 for X = Br and n = 1, 3/2 or 2 for X = I) were prepared and characterised by melting point, elemental analysis, thermogravimetric studies and infrared spectroscopy. From the enthalpies of dissolution in ethanol or 50% 2.4 N HCl in ethanol of the complexes, arsenic trihalides and ligands at 298.15 K, the standard enthalpies (DELTA-(r)H-kelvin) for the Lewis acid/base reactions were determined. From the DELTA-(r)H-kelvin values, the standard enthalpies of formation of arsenic trihalides and the standard enthalpies of formation of the ligands, the standard enthalpies of formation of the complexes (DELTA-(f)H-kelvin) were calculated. The standard enthalpies of decomposition of the complexes (DELTA-(D)H-kelvin), as well as the lattice standard enthalpies (DELTA-(M)H-kelvin) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (DELTA-(r)H-kelvin(g)), were calculated by means of thermochemical cycles, and the mean standard enthalpies of the arsenic-oxygen bonds (DBAR (As-O)) were estimated. The mean energies of the As-O bonds range from 98 to 125 kJ mol-1 in these complexes. The thermochemical data suggested the order AsCl3 > AsBr3 > AsI3 for acidity and gamma-piCONO > pyNO > alpha-piCONO for basicity. Correlation between several of the thermochemical parameters, and between the thermochemical parameters of these complexes and parameters of the complexes of non-oxidized ligands, were established.181143154