The concept of an efficient temporal transformation is introduced in solving stiff space-time equations encountered in nuclear reactor transients analysis. The multigroup diffusion equations are employed for the basic system description. Approximate solutions are found analytically and corrections are made using the alternating direction implicit method to solve the finite difference equations resulting from the transformation. The conditions for stability and convergence of this technique are discussed and the method is illustrated by a two-group two-dimensional analysis of a CANDU-BLW nuclear reactor cell. This method described here appears particularly appropriate immediatel following system perturbations but before the dominant temporal trend has been established.