NOVELTY - A biaxially oriented polyester film comprises polyester and at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, where the monocarboxylic acid has 5-50 carbon atoms, where the hydrolysis stabilizer is present in the film in the form of its reaction product with at least some of the end-groups of the polyester, and where the reaction product is obtained by the reaction of the hydrolysis stabilizer with the end-groups of the polyester in the presence of a metal cation selected from Group I and Group II metal cations.    USE - The biaxially oriented polyester film is useful as a layer in a photovoltaic cell (claimed).    ADVANTAGE - The biaxially oriented polyester film exhibits improved hydrolysis resistance, mechanical properties, dimensional stability and optical properties.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a photovoltaic cell comprising front-plane, electrode layers, a photovoltaic-active layer, and a back-plane, where the front-plane and/or the back-plane comprises the film;    (2) a process for the manufacture of the biaxially oriented polyester film, comprising: (i) extruding a layer of molten polyester and at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, where the monocarboxylic acid has 5-50 carbon atoms, where the hydrolysis stabilizer is present in the extrudate in the form of its reaction product with at least some of the end-groups of the polyester, and where the reaction product is obtained by the reaction of the hydrolysis stabilizer with the end-groups of the polyester in the presence of a metal cation selected from Group I and Group II metal cations; (ii) quenching the extrudate; (iii) stretching the quenched extrudate in two mutually perpendicular directions; and (iv) heat-setting the film, preferably at a temperature in the range of from stabilized by heat-setting at a temperature within the range of 200-225 degrees C;    (3) a method of improving the hydrolysis resistance of the biaxially oriented polyester film, comprising the step of reacting the polyester with at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, where the monocarboxylic acid has 5-50 carbon atoms, where the hydrolysis stabilizer is present in the film in the form of its reaction product with at least some of the end-groups of the polyester, where the reaction product is obtained by the reaction of the hydrolysis stabilizer with the end-groups of the polyester in the presence of a metal cation selected from Group I and Group II metal cations; and    (4) use of a metal cation selected from Group I and Group II metal cations for improving the hydrolysis resistance of the biaxially oriented polyester film comprising at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, where the monocarboxylic acid has 5-50 carbon atoms, where the hydrolysis stabilizer is present in the film in the form of its reaction product with at least some of the end-groups of the polyester, and where the reaction product is obtained by the reaction of the hydrolysis stabilizer with the end- groups of the polyester in the presence of the Group I or Group II metal cations.