Adsorption of organic phosphines on acidic surfaces produces phosphonium ions, which can be detected by P-31 NMR, since their chemical shift is different from that of the free phosphine. In the presence of excess adsorbed phosphine, lines from both phosphine and phosphonium are observed. Two-dimensional exchange NMR is applied to trimethyl- and triphenylphosphines adsorbed on SiO2-Al2O3 and demonstrates that free phosphine and phosphonium ions are undergoing chemical exchange at ambient temperatures. There are two populations of PMe3H+ which exchange with different rate constants, 7 x 10(4) and 800 s(-1) respectively. PPh3H+ exchanges much more slowly with PPh3, having a rate constant of 14 s(-1).