We propose a simple heteronuclear decoupling scheme for spin-1/2 nuclei under magic-angle sample spinning (MAS). The sequence, called XiX, consists of windowless rf irradiation with a repeat of two pulses of equal width, phase-shifted by 180degrees. The XiX scheme is found to be, within certain limits, insensitive to the flip angle of the individual pulses but only sensitive to the pulse width in units of the rotor period. It is therefore easier to implement and optimize than previously used sequences. The new sequence can lead to a significant improvement in decoupling over published sequences, in particular at high MAS frequencies. (C) 2002 Published by Elsevier Science B.V.