A process for producing poly(ester carbonates) and polyesters by reacting an aromatic or cycloaliphatic dicarboxylate salt such as sodium terephthalate with phosgene in an organic solvent in the presence of a catalytic amount of a tertiary nitrogen compound such as pyridine, optionally separating the suspended by-product inorganic salt and then interfacially reacting with a bisphenol such as bisphenol A. Excess phosgene in the diacid chloride synthesis can be carried over to the polymerization if a poly(ester carbonate) is to be produced or removed if a polyester is to be produced. Certain low levels of a tertiary nitrogen compound with a pyridine nucleus in the organic solution do not interfere with the interfacial polymerization.