The microstructure and permeability of rehydrated 20-100 mum thick partially coalesced (vinyl-actetate acrylic copolymer) SF091 latex coatings and a 118 mum thick model trilayer biocatalytic coating consisting of two sealant SF091 layers containing a middle layer of viable E. coli HB101 + latex were studied as delaminated films in a diffusion apparatus with KNO3 as the diffussant. The permeability of the hydrated coatings is due to diffusive transport through the pore space between the partially coalesced SF091 latex particles. Coating microstructure was visualized by fast freeze cryogenic scanning electron microscopy (cryo-SEM). The effective diffusion coefficient of SF091 latex coatings (diffusive permeability/film thickness) was determined as the ratio of the effective diffusivity of KNO3 to its diffusivity in water (D-eff/D). Polymer particle coalescence was arrested by two methods to increase coating permeability. The first used glycerol with coating drying at 4 degreesC, near the glass transition temperature (T-g). The second method used sucrose or trehalose as a filler to arrest coalescence; the filler was then dissolved away. D-eff/D was measured as a function of film thickness; content of glycerol, sucrose, and trehalose; drying time; and rehydration time. D-eff/D varied from 3 x 10(-4) for unmodified SF091 coatings to 6.8 x 10(-2) for coatings containing sucrose. D-eff/D was reduced by the flattening of latex particles against the surface of the solid substrate, as well as by the presence of the colloid stabilizer hydroxyethylcellulose (HEC). When corrected for the flattened particle layer, D-eff/D of HEC-free coatings was as high as 0.20, which agreed with the value prediced from analysis of cryo-SEM images of the coat surface. D-eff/D decreased by one-half in approximately 5 days in rehydrated SF091 coatings, indicating that significant wet coalescence occurs after glycerol, sucrose, or trehalose are leached from the films. D-eff/D of SF091 latex trilayer coatings containing viable E. coli HB101 cells decreased as cell loading was increased from 2.2 x 10(-2) for 64 g dry cell weight per liter of coat volume to 5 x 10(-3) for 151 g DCW/L of coat volume. The reduction in coating permeability with increasing cell loading is predicted by Maxwell's equation for D-eff/D in periodic composites.