Clinical procedures, such as acid etching and reshaping of the teeth supporting removable partial dentures by grinding off some enamel surface, increase the permeability of dental enamel. Teeth take several months in vivo to partially recover from such damage. In the meantime, the tooth is more susceptible to carious decay. To prevent this the ground or etched enamel should be effectively protected. Using electron paramagnetic resonance (EPR) and a two-chamber diffusion cell the authors studied the influence of adhesive resin applied to the ground and acid-etched enamel surfaces on the diffusion of spin label TMAPO (2,2,6-6 tetramethyl-4-acetamido-piperidine-1-oxyl) molecules through the enamel. The enamel permeability was measured in samples exposed to 1-min etching with 37% phosphoric acid, in samples etched for 5 min, and in samples ground with a diamond bur. Next, all the treated enamel surfaces were coated with Scotchbond Multi-Purpose Plus(R) dental adhesive system and the permeability measurements repeated. Scanning electron microscopy (SEM) was used to study the porosity of enamel surfaces. The adhesive resin film covering the etched or ground enamel surfaces was found to decrease significantly the diffusion through dental enamel. This finding confirms the clinical value of dental adhesives used to protect ground or accidentally acid-etched enamel surfaces. SEM analysis showed that adhesive resin covers the porous surface of the acid-etched and ground enamel tightly.