A nearly fully-dense (>98%) electroconductive silicon nitride - 27vol% hafnium diboride composite - was prepared by Hot Isostatic Pressing (HIP). A sintering temperature of 1710degreesC is required to restrain the decomposition of the silicon nitride phase, but this temperature is too low to obtain a full densification. The necessary presence of sintering aids (2wt% Y2O3 + 1wt% Al2O3) induced reactions between the phases. The only noticeable secondary phase formed by reaction between the initial powders (and detected by XRD and TEM after sintering), was the formation of a Y2Hf2O7 phase, (Y2O3*2HfO(2)). The oxidation tests were carried out under pure flowing oxygen (10L/h) between 900 and 1400degreesC for 24 hours. The composite material started to oxidize at 800degreesC but the weight gain was low up to 1400degreesC. From 900 to 1200degreesC, the rate of oxidation decreased with time. The oxides HfO2, B2O3 and a borosilicate glass were formed. At higher temperature (1200-1400degreesC), SiO2 and HfSiO4 appeared, and played an increasingly important role. At 1400degreesC, the whole oxidized sample was covered with a silicon oxide glassy phase while Yttrium and Hafnium were detected near the surface by EDS analysis, suggesting the presence of the mixed oxide Y2Hf2O7. The HfSiO4 phase occasionally formed on the surface large hollow spheres, with diameters greater than 50 mum while, in cross section, a very porous and titanium-depleted sublayer was observed. Since the silicon oxide glassy phase acted as a protective coating, this composite has promise for high-temperature applications.