Zero order release of active ingredients, physically dispersed in the glassy hydrogels has been ascribed in the past to the case II transport of the penetrant medium into the glassy matrix (phase erosion). Surface erosion of the polymer by dissolution has also been shown to result in the zero order release of the dispersed/dissolved active ingredient. This communication reports the zero order release of the pendent chain-linked p-nitro benzoic acid and 2,4-dinitro benzoic acid from glassy as well as swollen hydrogel matrices. The results have been elucidated on the basis of increase in the diffusivity of the active ingredient released by hydrolysis and the concomitant increase in the degree of swelling of the polymer matrix. This is the first reported finding of zero order release of an active ingredient from the matrix device as a result of bulk erosion.