Polycondensation of 1,10-decanediol with dimethyl-H-phosphonate taken in excess leads to oligomers with methyl-H-phosphonate end groups. The polytransesterification of the resulting oligomer as well as the related model reactions were studied. The synthesis of poly(decamethylene-H-phosphonate) was analyzed and the final product had (M) over bar(n) = 1.4-1.9 10(4) (from end groups, vpo, and (M) over bar(n) of the derived polymers). The exchange of the ester groups between two homoesters (dimethyl and diethyl phosphonates) used as models, conducted at r and catalyzed by metal alkoxide provides mixed (hetero) ester in a few minutes. If the concentration of the catalyst is not high enough, then the reaction does not go to equilibrium, because the alcoholate anions are converted into the anions of monoesters of the H-phosphonic acid, catalytically inactive at this temperature. However, these monoesters become catalytically active at higher temperature, i.e., at the conditions used for preparing higher molecular-weight products by transesterification. The apparent rate constants ((k) over bar) of the ester exchange catalyzed by monoester salt (modeling the propagation step in polytransesterification) were determined by two independent methods; at 130 degrees C (k) over bar similar to 1.0.10(-2) mol(-1).L.s(-1). The detailed study of the model polytransesterification, and particularly of the polymer end groups appearance and disappearance (studied by H-1-, C-13-, and P-31-NMR) allowed postulation of the reaction mechanism and confirmed our previous work, describing formation at these conditions of polymers with (M) over bar(n), > 10(4).