The use of two complementary techniques-based on a chromophore-labeled substrate and a fluorescently labeled enzyme-have provided a self-consistent picture of the role of soil in protecting amylase from deactivation by a peroxygen bleach. Amylase in solution is completely deactivated by the peracid N,N-phthaloylaminoperoxycaproic acid (PAP) via a mechanism which consists of a rapid initial step followed by a much slower second step. Over 80% of the enzyme activity is lost, under the conditions of the experiment, within 10 min of exposure to PAP. Amylase achieves steady state adsorption on to granular potato starch within a period of about 5 min. On retrograded starch films, this state is apparently reached within 60 s. The physical state of the substrate associated amylase is one of adsorption, with the enzyme not entering the soil matrix on the time scale of our experiments. Once adsorbed, amylase is partially protected against the effects of a peroxygen bleach due to the apparent inaccessibility of the active site coupled with the fact that a fraction of the adsorbed enzyme is resistant (on the time scale of our experiment) to being rinsed off by solvent. This fraction of enzyme is sensibly invariant with respect to adsorption time or adsorption level and is thought to be controlled by the heterogeneous nature of the substrate/amylase interaction.