Abstract: Unsaturated diesters are produced by a process which comprises reacting, in a flowing gaseous phase, a gaseous mixture comprising a conjugated diene, a carboxylic acid and molecular oxygen in the presence of (A) a catalyst comprising palladium, vanadium, antimony and at least one alkali metal carboxylate, or in the presence of (B) a catalyst comprising palladium, vandium, antimony, at least one alkali metal carboxylate and at least one alkali metal halide. The catalyst (A) maintains a high activity and a high selectivity for a long time. The catalyst (B), in which further the alkali metal halide is added, shows a higher activity and a higher selectivity for a longer time. And its activity and selectivity are stable and durable at high temperatures such as 160.degree. - 200.degree. C. Furthermore, the addition of the alkali metal halide suppresses combustion reaction of dienes such as butadiene remarkably.