Although chemical shift values of triethylphosphine oxide (TEPO) adsorbed on acidic solids have been considered as an indication of acid strength, in this work we demonstrate that the chemical shift depends also on the adsorbed amount of TEPO. On silica, the presence of three different adsorbed species, physisorbed on non-acidic surface, chemisorbed through a single H bond and chemisorbed through two H bonds, can be detected by the correlation of the 31P chemical shift with the TEPO adsorbed amount. TEPO chemical exchange between the different sites is demonstrated by the single NMR signal obtained in all the cases, and also by the variation of the line width, which is broader at low surface coverage due to the slower chemical exchange because of the longer average distance between surface sites.