Specimens of high purity polycrystalline titanium diboride, TiB2, were tested in uniaxial compression under vacuum at a strain rate of 5 × 1O−4sec−1 to determine the plastic yield behaviour. Generally, plastic deformation was detected only above 170O° C. The apparatus was able to apply u stress of 900 MPa at a maximum temperature of 2000° C. The yield stress data fit an Arrhenius function, with an apparent activation energy of O.8 eV atom−1. Dislocation glide over the Peierls barrier is thought to be the deformation mechanism, The dependence of the yield stress on the grain size obeyed the Hall-Petch relation within the bounds of experimental error. A TiB2 single crystal containing TiC precipitates was also compression - tested at 2000° C, and its yield stress was approximately four times the stress predicted by the Hall-Petch expression for e pure TiB2 single crystal, suggesting that the precipitate raises the yield stress but that the intrinsic lattice resistance is still significant. Submicrometre-sized graphite inclusions were observed in the polycrystalline specimens, but are thought not to have a direct effect on the yield stress in the temperature regime of the present study.