Abstract: A continuous method of carbonylating an epoxide and/or lactone with carbon monoxide with improved catalyst efficiency and reactor productivity is comprised of reacting the epoxy and/or lactone in a solvent with carbon monoxide in the presence of a catalyst at a temperature of at least 80° C. and an amount of water that is at most about 150 ppm of the effluent from the reactor. The amount of water in any of the ingredients used in a method of the invention is desirably substantially below the aforementioned water concentration in the effluent from the reactor. Likewise, in a method of the invention, the amount of polyether byproduct is substantially absent. The methods may be performed without recycling of the catalyst.