In this study the production Of Si3N4-B4C Composites using spark plasma sintering technique was studied. This technique was preferred in order to minimize the reactions between starting constituents. Fully dense Si3N4 ceramic was obtained by SPS. The bulk densities of all the produced composites was found to be significantly less than that Of Si3N4, and this was attributed to the formation of the relatively low-density reaction products. In the B4C-containing composites there was a trend for the bulk density to increase slightly with decreasing particle size of B4C powder. Composites containing both B4C and TiO2 had somewhat higher bulk densities when compared with those just containing B4C. This was related to the formation of relatively high-density reaction products in significant amounts. The fine B4C added to the Si3N4 base composition containing Al2O3 and Y2O3 as sintering additives, even when incorporated in significant amounts, was consumed readily during the fast sintering process. Using coarse B4C particles reduced the reaction kinetics to some extent. As a result of reactions between Si3N(4) and B4C particles SiC, h-BN and metallic Si were formed. When both B4C and TiO2 were added together additional phases of Ti (C, N) and TiB2 were formed. Possible reactions that explain the formation of the in-situ phases were proposed through thermodynamic considerations. (c) 2008 The Ceramic Society of Japan. All rights reserved.