Type Ia and type IIa diamonds have been heated to temperatures in the range 2000-2300 degrees C under a pressure of about 4.8 GPa. The changes in the defect structure of these diamonds as a result of the heat treatment have been examined by optical absorption measurements, integrated X-ray spike measurements and by electron microscopy. Type Ia diamonds changed colour from colourless to yellow after being heated for 1 min in the temperature range 2250-2300 degrees C and it has been shown that the yellowing was due to scattering. The scattering centres responsible were elliptically shaped cracks in the cube planes with their longest dimension in a $\langle $100 $\rangle $ direction. It is proposed that these cracks emanated from the platelets which are present in this type of diamond, owing to the breaking of carbon-carbon bonds at the periphery of the platelets where there is a tensile strain present. It is very probable that the cracks contained amorphous or partially crystallized carbon. Evidence is also presented for bond breaking and crack formation at edge dislocations in type IIa diamonds as a result of the treatment with the crack again filled with amorphous or partially crystallized carbon. Type IIa diamonds did not turn yellow as a result of the treatment. Prolonged heating of both types of diamond resulted in a catastrophic reduction in strength due to the growth of the cracks. Integrated X-ray spike intensity measurements and electron microscopy observations showed no apparent change in the structure of the platelets as a result of heating type Ia diamonds in the temperature range 2200-2250 degrees C. However, the optical absorption measurements indicated that changes were taking place at the platelets. It is proposed that nitrogen atoms diffused away from the platelets and combined with other point defects to produce colour centres. It is also proposed that the apparently unchanged structure of the platelets was because the nitrogen is present in platelets as a minority species and the platelets consist mostly of segregated interstitial carbon atoms with the nitrogen atoms dispersed, probably at random, in the platelets. Localized bond breaking also probably occurred at the periphery of the platelets as a result of heating in this temperature range.