Poly(amido-carbonate)s and poly(amido-thiocarbonate)s derived from 1,6-bis(4-hydroxyphenyl-carbamoyl)-hexamethylene I and 1,4-bis(4-hydroxyphenyl-carbamoyl)-phenylene II, and phosgene or thiophosgene, have been synthesized by a phase transfer catalysis process, which was ineffective for polymers derived from monomer I because the best results were obtained without catalyst, but was effective for those derived from monomer II obtaining higher yields when the catalysts were used. The ineffectivity was attributed to a hydrolytic process of the polymeric chains in the reaction media.