The effect of a commercial ion exchange paste on the tensile strength of high- and low-expansion dental porcelains was investigated. Strength measurements were correlated with changes in surface chemistry. The paste was found to be more effective for aluminous porcelains than for an inlay ceramic, although only when applied to the surface placed in tension. Surface finish also influenced the effectiveness of the paste. The depth of ion exchange was assessed by energy dispersive x-ray analysis and was found to extend to at least 100 microns, although the most marked change occurred within a 10-microns layer below the surface.