This final paper in a series of three uses transmission and scanning electron microscopy to compare the category III dentinal adhesive systems-those with shear bond strength values near or exceeding 17 MPa. Contemporary commercial dentinal adhesive systems such as Superbond and Scotchbond Multi-Purpose, chosen for their varied chemical pretreatments, are contrasted; a contemporary commercial system (Prisma Universal Bond 3) with characteristics of category I and II dentinal adhesives is also included for comparison. The shear bond strength values attained with most category III systems are high enough to cause cohesive failure of the dentin during bond strength testing. This result is attributed to a combination of factors that include effective wetting and penetration of the prepared dentinal surface as well as a tendency to leave collagen fibers at the adhesive-dentin interface in an apparently structurally intact state.