An enzyme assay for tannin is described. It is based on the following steps: (i) bovine serum albumin (BSA) is absorbed onto polystyrene microplates; (ii) tannin is bound to the BSA-coated plates; (iii) alkaline phosphatase is then interacted with the free tannin-binding sites. The method takes advantage of the multiple hydroxyl groups of tannin which can associate more than one ligand, e.g., proteins. A pH-dependent dynamic equilibrium sets up between bound and unbound tannin and is controlled only by its initial concentration.