Whilst there is merit in using NH3 or CO as probes to determine the surface acidity of solid catalysts by thermal and/or spectroscopic means, we show that, following Wakabayashi et al., quantitative spectroscopic changes accompanying the adsorption of dinitrogen offer more reliable assessments. Subtle differences arising from the use of N-2 as between CoALPO and zeolitic (aluminosilicate) catalysts are also discussed.