Homogeneous catalytic oxidation of but-1-ene by oxygen in molten potassium hydrogensulfate, at 220 degreesC and P(H2O) = 0.04 atm, is reported. Experimental and theoretical studies of the electroactivity range of the solvent confirm that the latter is oxidized to oxygen and reduced to hydrogen. Potentiometric studies of the vanadium system, which takes part in the catalysis, show the existence of VO3+ in very acidic medium. Solubilities of vanadium(IV) and the acidity constants of vanadium species are determined. Kinetics studies of various reactions taking place during the oxidation of but-1-ene by oxygen are reported. But-1-ene is oxidized by the solvent which is regenerated by vanadium(V), the catalyst being a weaker oxidizing agent than the solvent. Experimental conditions for catalysis are analyzed. The low temperature (220 degreesC) favored the use of gas mixtures for which there is no risk of combustion.