AbstractThe identification of acid and nonacid species at the external surface of zeolites remains a major challenge, in contrast to the extensively‐studied internal acid sites. Here, it is shown that the synthesis of zeolite ZSM‐5 samples with distinct particle sizes, combined with solid‐state NMR and computational studies of trimethylphosphine oxide (TMPO) adsorption, provides insight into the chemical species on the external surface of the zeolite crystals. 1H–31P HETCOR NMR spectra of TMPO‐loaded zeolites exhibit a broad correlation peak at δP ∼35–55 ppm and δH ∼5–12 ppm assigned to external SiOH species. Pore‐mouth Brønsted acid sites exhibit 31P and 1H NMR resonances and adsorption energies close to those reported for internal acid sites interacting with TMPO. The presence of an external tricoordinate Al‐Lewis site interacting strongly with TMPO is suggested, resulting in 31P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites.