We show that appropriate modulation of the rf amplitude in the two-pulse phase-modulation (TPPM) H-1 decoupling sequence provides an appreciable improvement in the decoupling efficiency. The effect of the amplitude modulation on TPPM is examined using the average Hamiltonian theory, and the improvement in the decoupling efficiency is ascribed to third averaging. Improved decoupling is demonstrated with C-13 magic-angle spinning (MAS) NMR in glycine and uniformly C-13, N-15-labeled glycylisoleucine.