Two-dimensional ¹H–³¹P heteronuclear correlation NMR of trimethylphosphine oxide (TMPO) adsorbed in zeolites, in tandem with DFT calculations, challenges previous one-dimensional ³¹P NMR assignments, enabling the unambiguous discrimination of Brønsted and Lewis acid sites, extending the understanding of TMPO:Brønsted complexes formed with distinct stoichiometries at the HZSM-5 zeolite surface, and the proton-transfer mechanism.