Abstract: A process for making a butyl ester from butadiene by reacting butadiene or a hydrocarbon fraction containing butadiene with a saturated aliphatic monocarboxylic acid, wherein the catalyst is rhenium(VII) oxide or an organic sulphonic acid containing at least 2 sulphonic acid groups per molecule wherein the ratio of the number of carbon atoms to the number of sulphonic acid groups in the organic sulphonic acid is in the range 1:1 to 1:0.15. Preferred catalysts are organic disulphonic acids for example ethane-1,2-disulphonic acid. The process can be used for making unsaturated esters, or, by hydrogenation of the product, for making saturated esters such as for example butyl acetate. Catalyst can be purified and recycled to the reactor.