Since the strength of the filler-matrix boundaries largely determines the properties of composite materials, the study of the interaction between the filler and the matrix is an urgent task for the development of high-performance composites. The purpose of this work is to study the products of contact interaction between the diamond and the matrix material of wear-resistant diamond-metal composites obtained by explosive pressing with subsequent heat treatment. Composites structure: natural diamond powder in a two-component iron-carbon matrix. Structural and phase studies of the diamond-matrix interface were performed using optical microscopy, scanning electron microscopy, and x-ray analysis. It is revealed that a significant factor contributing to the improvement of the performance properties of experimental diamond composites is the formation of a boron-carbide phase at the interfacial boundary with high values of strength and wear resistance. The scientific novelty of the work is due to the fact that the problems connected with a formation and a state of matrix-filler zones of various composite materials are one of the main ones in the field of fundamental research of the crystalline multiphase state of matter. The obtained data can be used in the engineering practice in the development of new composite materials.