Polymerization of lactones such as delta-valerolactone (VL) and epsilon-caprolactone (CL) with protic compounds proceeded in the presence of an aluminosilicate mesoporous zeolite Al-MCM-41 (pore diameter of 27 Angstrom, surface area = 1010 m(2) g(-1), and Si/Al = 17), to give polyesters with a narrow molecular weight distribution. A sequential two-stage polymerization of VL and CL with Al-MCM-41/butanol gave a block copolymer. NMR and GPC studies showed that the produced polymer carries a terminal group originating from the protic compound. In contrast with Al-MCM-41, pure silicate MCM-41, an MCM-41 with masked SiOH functionalities, and a "microporous" aluminosilicate with much narrower pores (zeolite-Y; pore diameter = 8 Angstrom) were not effective for the polymerization under similar conditions. Infrared spectroscopy of an inclusion adduct of Al-MCM-41 with VL suggested a possible interaction of the monomer onto the aluminosilicate surface. Al-MCM-41, isolated from the polymerization mixture, could be used again for the polymerization.