A method based on nonlocal density functional theory (NLDFT) has been used to interpret the data for the adsorption of nitrogen at 77 K within the pores of three different commercial fluid cracking catalysts (FCCs) before and after their use in a refinery catalytic cracking unit. The integral equation of adsorption was inverted by a regularization method to yield the micropore and mesopore size distribution over a wide range of pore widths. The results obtained are compared with the results of more traditional data treatments and indicate that the NLDFT model can provide a realistic pore volume and surface area estimation in commercial FCCs irrespective of their chemical composition and pore width distribution. Both BET and Langmuir methods grossly underestimate the FCCs surface area, and only the NLDFT method yields reliable surface area and pore volume measurements over the entire micro-meso porosity range present in the cataysts under study. Adsorption microcalorimetry results using ammonia as a probe molecule reveal that as long as Lewis acid sites with strength greater than 100 kJ/mol are present and as long as these sites are available to gas oil, FCCs can retain their useful cracking activity and selectivity properties.