Transverse dephasing times T-2' in spin-echo MAS NMR using rotor-synchronised Hahn-echo pulse-train (RS-HEPT) low-load H-1 decoupling are evaluated. Experiments were performed at 300 and 600 MHz for (CH)-C-13-labelled L-alanine and (NH)-N-15(delta)-labelled L-histidine center dot HCl center dot H2O, together with SPINEVOLUTION simulations for a ten-spin system representing the crystal structure environment of the (CH)-C-13 carbon in L-alanine. For 30 kHz MAS and nu(1)(H-1) = 100 kHz at 300 MHz, a RS-HEPT T-2' value of 17 +/- 1 ms was obtained for (CH)-C-13-labelled L-alanine which is similar to 50% of the XiX T-2' value of 33 +/- 2 ms. Optimum RS-HEPT decoupling performance is observed for a relative phase of alternate RS-HEPT pi-pulses, Delta phi = phi' - phi, between 40 and 60 degrees. For experiments at 600 MHz and 30 kHz MAS with (CH)-C-13-labelled L-alanine, the best RS-HEPT (nu(1)(H-1) = 100 kHz) T-2' value was 3 times longer than that observed for low-power continuously applied sequences with nu(1)(H-1)