The 200-300 mu m soft zone of dentin, found beneath enamel in crowns of human teeth, is thought to fulfill important roles in tooth function. but little is known about its structure-mechanical relations. Scanning electron microscopy images of fracture Surfaces showed that near the dentino-enamel junction (DEJ), a porous reticulate matrix of intertubular-dentin contains tubules with no peritubular lining. Peritubular-dentin however is found at some distance from the DEJ, and it gradually thickens kith increasing depth into the bulk dentin. Concurrently, tighter packing of the collagen fibers is observed with a gradual increase in mineral deposits on and between the fibers. This structurally graded zone is known to be softer when tested for rnicro-hardness. It undergoes greater strain compared to bulk dentin, when measured using Moire interferometry. We investigated the deformation and stiffness of this zone by means of non-contact laser-speckle interferometry (ESPI), and nanometer-scale deformations were tracked during compression-testing performed in water. We report a significantly reduced stiffness of this zone compared to bulk dentin. with mid-buccal regions or teeth averaging 3.5 GPa compared with 9.7 GPa in mid-lingual regions. Our results support and expand upon the hypothesis that the durability of the whole tooth relies upon a bucco-lingual asymmetric matching of stiffness by means of all interphase: a cushioning soft layer between enamel and bulk dentin. (C) 2005 Elsevier Inc. All rights reserved.