Calorimetric measurements have been performed on the melt-gas systems M2S2O7-V2O5 and M2S2O7-M2SO4(Sat)-V2O5 (M = K and Cs) in SO2(g) and Ar(g) at 430-480 degreesC and for mole fractions X-V2O5 = 0-0.1. Enthalpies have been obtained for complex formation reactions as (VO)(2)O(SO4)(4)(4-) +SO42- reversible arrow 2VO(2)(SO4)(2)(3-) +S2O72- for V(V) and 2VO(SO4)(2)(2-)+ 3SO(4)(2-) +SO3 reversible arrow 2VO(SO4)(3)(4-) +S2O72- for V(IV). Similarly enthalpies of the V(V)-V(IV) redoxreactionS (VO)(2)O(SO4)(4)(4-) + SO2 reversible arrow 2VO(SO4)(2)(2-) + SO3 and 2VO(2)(SO4)(2)(3-) +SO42- + SO2 reversible arrow 2VO(SO4)(3)(4-) could be calculated. These enthalpies of reduction compare well with those obtained by spectrophotometric measurements in the same temperature range.