An alkylene carbonate is produced by the reaction of an alkylene oxide with carbon dioxide in the presence of a phosphonium compound as catalyst in a process in which (a) the alkylene oxide, carbon dioxide and the phosphonium catalyst are continuously introduced into a reaction zone from which a product stream containing alkylene carbonate and catalyst is withdrawn, (b) alkylene carbonate and a mixture of alkylene carbonate and phosphonium catalyst are separated from the product stream, (c) the alkylene carbonate, separated in step (b), is recovered as product, and (d) the mixture of alkylene carbonate and phosphonium catalyst is continuously recycled to the reaction zone. The alkylene carbonate thus produced is suitably reacted with an alkanol to produce an alkane diol and a dialkyl carbonate.