A new method is presented for decoupling spins with S = 1 like D in anisotropic media while observing other spins such as residual protons in partly deuterated samples. The carrier frequency of a weak radiofrequency (RF) field is applied near the center of the doublet arising from the quadrupolar interaction of the S = 1 spins. The phase of the RF field is periodically reversed with intervals matching the reciprocal of the magnitude of the quadrupolar splitting. It is shown by theory and expt. that, even when the irradiating field is quite weak, the efficiency of this phase-alternated decoupling scheme is much better than for simple continuous-wave irradn. at the center of the doublet, an established technique which is usually referred to as double quantum decoupling. The phase-alternated expt. makes it possible to decouple large quadrupolar interactions with a weak RF field. A theor. anal. and numerical simulations are presented to demonstrate the decoupling performance. Proton spectra of partly deuterated dibromobutane in a liq. cryst. solvent were recorded to illustrate the efficiency of phase-alternated D decoupling. [on SciFinder (R)]