The reaction of (Bu-t)(2)ArP (la-h), where the para position of the Ar group contains electron donating or electron-withdrawing groups, with sulfated zirconium oxide partially dehydroxylated at 300 degrees C (SZO(300)) forms [(Bu-t)(2)ArPH][SZO(300)] (2a-h). The equilibrium binding constants of la-h to SZO(300) are related to the pK(a) of [(Bu-t)(2)ArPH]; R3P that form less acidic phosphoniums (high pKa values) bind stronger to SZO(300) than R3P that form more acidic phosphoniums (low pKa values). These studies show that Bronsted acid sites on the surface of SZO(300) are not superacidic.