A series of sulfate-promoted ZrO2 solid acid catalysts samples were prepared, each sample differing only in the pH of the sulfuric acid soaking step of the synthesis procedure. Fourier-transform infrared (FTIR) spectra and thermal weight-loss measurements (as measured by thermal gravimetric analysis (TGA)) indicate that the pH of the sulfuric acid soaking solution influences the number of surface proton sites. The activity of sulfate-promoted ZrO2 in catalyzing the isomerization of n-butane to iso-butane at 220 degrees C and the surface area of the calcined catalyst are also found to depend on the soaking solution pH, with maximum surface areas and activities observed between pH 6 and pH 9. Proton nuclear magnetic resonance (NMR) chemical shifts measured using combined rotation and multiple pulse spectroscopy indicate that the sulfating step produces proton species that are more acidic than proton sites on the nonsulfated support. The proton NMR spectra of these samples consist of at least two overlapped, but distinct, resonances that can be separated on the basis of a large disparity in spin-lattice (T-1) relaxation times. By comparing the experimental spectra with a series of simulated spectra, the long T-1 component has been assigned to a discrete three-or four-proton site that includes one water molecule, and the short T-1 component to an ill-defined cluster of protons. Variable-temperature NMR lineshape data also suggest that the protons are in rapid motion on the sulfate-promoted zirconium oxide surface.