A method of characterising relative proportions of magnetic resonance (MR) chemical species where one or more of the chemical species has a multi-peak MR spectra includes defining an in-phase measurement time window and an out-phase measurement time window relative to a frequency difference between the two dominant spectral peaks of at least one said multi-peak chemical species. The in-phase window captures the signal that is progressing to being maximally in-phase for the two dominant peaks, and the out-phase window captures the signal that is progressing to being maximally out of phase for the two dominant peaks. The method includes identifying an increased upward deviation in the signal over the in-phase window relative to a reference signal decay over the out-phase window, where such upward deviation in the signal over the in-phase window provides an indicator of the increased presence of the one or more multi peak spectral species over the other species.