A process for preparing saturated carboxylic acids having from 1 to 4
carbon atoms at a reaction temperature of from 100.degree. C. to
400.degree. C. and pressures of from 1.2.times.10.sup.5 Pa to
51.times.10.sup.5 Pa by gas phase oxidation of saturated and/or
unsaturated C.sub.4 -hydrocarbons, with an oxygen-containing gas and water
vapor in the presence of at least one catalyst. The gas leaving the
reactor is partly recirculated in a reaction gas circuit. This reaction
gas circuit is configured such that part of the organic acids formed in
the gas-phase oxidation is taken from the gas leaving the reactor so that
the acid content of the recirculated part of the gas leaving the reactor
is from 0.01% to 6.0% by volume.
