The absorption spectra of eight type I and three type IIa diamonds irradiated with neutrons, electrons or $\gamma $-rays have been recorded at 80 and 290 degrees K after various heat treatments in the temperature range 0 to 900 degrees C. It is found that heating in the range 350 to 450 degrees C causes a general reduction in the irradiation-induced absorption owing to the recombination of those interstitials and vacancies which are near neighbours. Heating type IIa diamonds at 600 degrees C causes a large reduction in the irradiation-induced lines and new lines appear. These are probably due to pairs of identical defects, and the kinetics of their formation during isothermal heating at 600 degrees C are presented. At higher temperatures all absorption lines in type IIa diamonds disappear and only continuous absorption remains. This is probably due to amorphous or graphitic regions produced by agglomeration of defects. Type I diamonds show the same kind of absorption, but in addition show an increase in strength of the natural lines and also some new absorption lines which are not removed by heating at 900 degrees C. It is suggested that these additional processes are due to the anchoring of vacant atomic sites and interstitial carbon atoms at crystal imperfections present only in type I diamonds.