Manufacture of polyesters

A process for the manufacture of a polyester, of any desired number average molecular weight, M.sub.n, less than about 20,000 as measured by size exclusion chromatography relative to polystyrene standards, characterised by the hydrolysis in homogeneous solution, with an aqueous acid, of a polyester of the same chemical composition as the desired polyester but having a higher number average molecular weight, M.sub.n.sup.o, measured as defined above, than the desired polyester product, for a period of time, t, determined from the equation:

• ##EQU2## wherein K is a constant which has been predetermined by a calibration experiment using the particular polyester, the particular water, acid and polyester concentrations, and the particular hydrolysis temperature desired.