Process for the heat stabilization of alkyl alkoxyalkylidenemalonates in the presence of Lewis acids, such as metal catalytic compounds employed as catalysts during the preparation of the alkyl alkoxyalkylidenemalonates by condensation of a suitable malonate with a suitable ortho ester. The condensation reaction mixture is heated in the presence of a stabilizing compound selected from the group consisting of 8-hydroxyquinolines and organic acid phosphates in an amount sufficient to stabilize the alkyl alkoxyalkylidenemalonates against thermal decomposition.