Monocrystals of sapphire and magnesia together with glass samples have been subjected to ion implantation prior to thermal shock testing in a plasma of continuously varied intensity. Measurements of the gap between adjacent fragments have been used to estimate the surface temperature. Fracture and deformation characteristics of the surface layer are measured in ion implanted and unimplanted samples using optical and scanning electron microscopy. Ion implantation is shown to modify the near-surface structure of ceramic samples by introducing damage, which makes crack initiation easier under thermal stresses in sapphire and magnesia crystals. The fracture threshold and maximum crack density are shifted towards the lower temperature range. Ion implanted MgO crystals show a ten fold increase in surface crack density. An increased crack density results in a decreased degree of damage characterised by the depth of crack penetration. The thermal stress resistance parameter of glass samples is increased at relatively small doses and decreased at higher doses. The results suggest that crack density and the degree of fracture damage in brittle ceramics operating under thermal shock conditions can be effectively controlled by ion implantation which provides crack initiating defects in the near-surface region. 22 refs., 8 figs.