Certain anion-modified metal oxides such as ZrO2/SO4 are acid catalysts but there is some controversy as to their superacidic strength. There is considerable doubt as to the validity of using Hammett indicators to characterize the strengths of these solid acids. It appears that a more reliable measure of their acid strength is needed. This paper proposes use of the cleavage of diphenylmethane (DPM) at ambient conditions, known to proceed via protolysis, as a convenient qualitative way to compare the relative protonic acid strengths of these strong solid acids. The activities of these solid acids were compared with that of 100% sulfuric acid, which did not cleave DPM, and two liquid superacids, viz., fuming sulfuric acid and triflic acid at 25 degrees C and atmospheric pressure. SnO2/SO4, TiO2/SO4, and ZrO2/WO3 were less active than ZrO2/SO4. ZrO2/MoO3, and Fe2O3/SO4 were inactive for DPM cleavage at ambient conditions. The role of Lewis acid sites in the cleavage of DPM at ambient conditions is likely to be negligible, so that this reaction appears suitable for comparing strengths of Bronsted acid sites present on these catalysts. The effect of temperature and hydrogen pressure on the activity of a sulfated zirconia catalyst in hydrocracking of DPM was also measured.