The efficiency of broadband heteronuclear decoupling has been greatly improved by recent innovations which use periodic sequences of composite pulses (I-S), for example, the MLEV (1,2) or WALTZ (6, 7) sequences. This has important implications for high resolution carbon13 spectroscopy, particularly at high magnetic fields. One consequence of the periodicity of the perturbation is that the observed spectrum contains weak sideband responses at integral multiples of the decoupler cycling rate, symmetrically disposed about the decoupled resonance line. These cycling sidebands are analogous to the well-known spinning sidebands that occur whenever the sample is rotated in an inhomogeneous applied field; indeed they may be thought of as arising from a magnetic field modulation transmitted through the spin-spin coupling (9). We would expect that all broadband decoupling schemes (10-13) would exhibit sidebands of this type, although the pseudorandom phase modulation used in noise decoupling (10) probably generates so many different modulation frequencies that the cycling sidebands become virtually indistinguishable from the baseline noise. This must eventually impose a limit on the sensitivity of the NMR experiment when broadband decoupling is employed. The purpose of this communication is to analyze this problem and to show that a simple modification of the decoupling sequences serves to reduce the amplitudes of cycling sidebands by an order of magnitude. Cycling sidebands are absent if the carbon-l 3 free induction signal is observed stroboscopically, that is to say, if the sampling operation is restricted to corresponding points in each decoupler supercycle (4,8). For example, at the end of each supercycle, the free induction signal is returned very close to the value it would have for perfect decoupling, but inbetween these points the signal deviates from this ideal value (14). Faster sampling brings to light this additional modulation, which, after Fourier transformation, appears as cycling sidebands. For the practical cases of interest, the carbon13 sampling rate must be much faster than the decoupler cycling rate, so it is not feasible to employ stroboscopic detection. However, the higher the level of B2, the faster the decoupler cycling rate and the weaker the cycling sidebands. When time averaging is used, some relief from this problem can be obtained by sampling asynchronously, that is to say, each transient starts at a different point in the decoupling cycle. Then, since some of the spurious modulation components have random phases they cancel after several accumulations. This is easily implemented by building a “black-box” decoupler (15) which is deliberately desynchronized with