The effects on strength and electrical conductivity of heat treatments in air at 800, 1000 and 1200 degreesC for 100 h were evaluated on hot pressed Si3N4-35 vol.% TiN composite. The long term oxidation involves microstructural changes at the surface and subsurface, such as the formation of oxide scales and of a multilayer microstructure under the oxide towards the bulk, due to the involved chemical reactions, the outward diffusion of cations (Ti, Y, Al) and impurities, and inward diffusion of oxygen during heat treatments. At the same time, crystallisation of the grain boundary phase occurs. The decrease of the room temperature (RT) flexural strength reached about 40% in the samples exposed at 1200 degreesC for 100 h, compared to the as-produced materials. The variation of the electrical resistivity was related to the microstructure modification due to oxidation: the surface oxide scale is an insulator and increases the electrical resistivity from 10(-3)-10(-4) to 10(10)-10(11) Omega cm. The effects induced by an annealing treatment in argon at 1200 degreesC for 100 h have also been investigated and compared to those caused by the oxidation. (C) 2002 Elsevier Science B.V. All rights reserved.