Abstract: In a method of heteronuclear decoupling in high resolution pulsed NMR spectroscopy, during acquisition of signals emanating from a nuclear species to be observed (e.g. carbon-13), irradiation of an interfering nuclear species (e.g. protons) is effected by means of a train of composite pulses, each of which approximately inverts the longitudinal magnetization. The pulses are of two types respectively having opposite r.f. phases, and the train constitutes a repeated sequence which consists of 2.sup.N+1 pulses of each type (where N is a positive integer) and which has a form chosen in accordance with specific rules to ensure effective decoupling.