[en] The transformation of F to R centres in KCl has been investigated by means of optical absorption and positron annihilation measurements. The intensity of the long-lived component (approximately equal to 1 ns) in the life-time spectra, which is due to annihilation in F centres, decreases with increasing R centre concentration. A simple model is proposed to explain the experimental data, from which it is indicated that annihilation in R centres takes place with a lifetime of only approximately 200 ps. The oscillator strength of the R₁ band is determined to be about 0.3. (author)