A sound knowledge of mechanical properties of rocks at high temperatures and pressures is essential for modelling volcanological problems such as fracture of lava flows and dike emplacement. In particular, fracture toughness is a scale-invariant material property of a rock that describes its resistance to tensile failure. A new fracture mechanics apparatus has been constructed enabling fracture toughness measurements on large (60 mm diameter) rock core samples at temperatures up to 750°C and pressures up to 50 MPa. We present a full description of this apparatus and, by plotting fracture resistance as a function of crack length, show that the size of the samples is sufficient for reliable fracture toughness measurements. A series of tests on Icelandic, Vesuvian and Etnean basalts at temperatures from 30 to 600°C and confining pressures up to 30 MPa gave fracture toughness values between 1.4 and 3.8 MPa m1/2. The Icelandic basalt is the strongest material and the Etnean material sampled from the surface crust of a lava flow the weakest. Increasing temperature does not greatly affect the fracture toughness of the Etnean or Vesuvian material but the Icelandic samples showed a marked increase in toughness at around 150°C, followed by a return to ambient toughness levels. This material also became tougher under moderate confining pressure but the other two materials showed little change in toughness. We describe in terms of fracture mechanics probable causes for the changes in fracture toughness and compare our experimental results with values obtained from dike propagation modelling found in the literature. © 2004 Published by Elsevier B.V.