Proton-releasing properties at the oxide/aqueous solution interface were examined as a function of pH for two series of mixed oxide aerogels and their pure oxide components using potentiometric titrimetry. Reports of synthesis, surface characterization, and catalytic activity (1-butene isomerization) have appeared recently for similar preparation batches of titania-silica and zirconia-silica. After deconvolution of potentiometric titration data, the pK spectra of the oxide surfaces were obtained; they reveal the number and strength of various proton donor sites present under "wet" conditions. The activity in l-butene isomerization measures weak Bronsted sites of the "pseudo-dry" catalyst (reaction at 423 K after drying at 473 K). We find that the activity for 1-butene isomerization is linearly related to the density of a particular type of proton donor site determined from pK spectra. This shows that only a limited number of the total Bronsted sites accessed by potentiometric titrimetry is specifically active for 1-butene isomerization under the conditions used here. The pK range of active sites and the corresponding TOF numbers are 6.3 < pK < 8.7, (8.2 +/- 2.1) x 10(-3) s(-1) for titania-silica and 6.4 < pK < 6.5, (93 +/- 24) x 10(-3) s(-1) for zirconia-silica. In the two catalysts series the catalytic performance varied with either composition (titania-silica) or preparation parameters (zirconia-silica) at constant composition.