Surface areas (BET) calculated from nitrogen sorption on aluminum oxide supports with varying MoO3 and WO3 loadings were consistently higher than surface areas calculated from mercury penetration data. Increasing the MoO3 or WO3 content from 0% to about 20% resulted in a gradual decrease in the surface areas calculated from mercury penetration even though the BET surface area remained constant. The mercury contact angles which provided agreement between BET and mercury penetration surface areas decreased with increasing molybdenum or tungsten oxide loading up to the concentration corresponding to about one monolayer coverage, and approached that of unsupported MoO3 or WO3. The best agreement of data from the two techniques was obtained for samples with pores in the 100-200 angstrom region.