A statistical model has been developed for the prediction of molecular weight and molecular weight distribution in transesterification reactions of diols onto polyesters (ideally polycondensated with a polydispersity of 2). The model was checked out on a transesterification reaction of PEG or diethylene glycol on PLLA, which is a simple and noncatalytic route for obtaining blends of low-molecular-weight block copolymers PEG-PLLA and low-molecular-weight PLLA, which have biomedical interest. From this mathematical model, it is possible to predict the change in molecular weight and molecular weight distribution as a function of the different experimental variables. Reactions were characterized by H-1 NMR and GPC.