Corrections pertaining to absolute disintegration rate determinations by the 4 pi BETA -- gamma coincidence method are discussed. It is demonstrated that because of the high efficiency of the 4 pi detector most corrections are small. In general, the largest corrections, apart from background, are those due to the accidental coincidence rate and dead time losses, expressions for which are derived. A 4 pi BETA -- gamma coincidence apparatus is described. An account of the performance of this instrument is given, together with tbe experimental verification of the accidental coincidence rate and dead time corrections. The performance suggests that accuracies of the order 0.1 per cent are now feasible. (auth)