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. Additionally, the catalysts (A) and (B), especially (B) can be easily reactivated when they lose activity.