The gel formation of hydrocolloid (sodium alginate and kappa-carrageenan) is the result of the molecule aggregation in presence of an effective cation. The gelling process was considered as a process of diffusion-reaction between the effective cation and the hydrocolloid to produce a porous solid structure.A simple mathematical model for predicting the reaction interface position in alginate or alginate-carrageenan gelation was developed. The model represents appropriately the physicochemical phenomenon and allows to determine the time for complete crosslinking of the hydrocolloid and the amount of incorporated ions. (C) 2003 Elsevier Science B.V. All rights reserved.