Abstract We have observed that the weight of κ-carrageenan hydrogel decreases in an oscillatory manner with time in distilled water. In order to explain this observation, a kinetic model for an oscillatory erosion of hydrogel is proposed in which two coefficients, α and β, are introduced for the initiation and the termination of the hydrogel erosion. Analysis of erosion data on κ-carrageenan hydrogel is attempted via the present model. Hydrogel starts to swell when transferred into an aqueous solution. The swelling continues until the weight W( t) of water in the hydrogel reaches αP( t), where P( t) is the polymer weight in the hydrogel. When W( t) = αP( t), erosion, that is, liberation of both polymer and water from the hydrogel, starts. The erosion process is terminated when the water weight in the hydrogel decreases to βP( t). The swelling and erosion take place alternately and the swelling/erosion cycle continues repeatedly until all the polymer molecules are liberated from the hydrogen matrix. It is found that these two parameters, α and β, affect both the amplitude and frequency of the oscillation. Further, the amplitude and the frequency of the oscillation depend on the water influx and efflux rates.