We have examined the acidic properties of a SiO2-on-Al2O3 monolayer catalyst by P-31 NMR of adsorbed tricyclohexylphosphine and triphenylphosphine. These large probe molecules permit the assessment of steric effects, by comparison with trimethylphosphine, and reveal differences in acid strengths of surface sites. We find that there are significant constraints to accessing Lewis sites, but not Bronsted sites. The concentration of strong Bronsted sites is slightly less than half of the total concentration of Bronsted sites.