A four-point bend test was used to determine the fracture toughness of mechanical grade and di-electric (optical) grade chemical vapour deposited (CVD) diamond. The validity of the test was first confirmed by measuring the toughness of alumina and confirming the results with literature values. The toughnesses of both types of CVD were similar; 8.5 +/- 1.0 and 8.3 +/- 0.4 MParootm respectively. This is higher than the value of 3.4 +/- 0.5 MParootm measured for good quality natural diamond by Field and Freeman, [ 1] using an indentation technique. It is suggested that this is primarily due to differences in surface roughness. There were enough samples to make a preliminary study of the effect of temperature and these data are reported. (C) 2004 Kluwer Academic Publishers.