The steady-state rate of n-pentane conversion has been related to the Bronsted acidity of dealuminated (D) acid (H) Y zeolites (DHY) and ZSM-5 (DHZ). Low-temperature CO adsorption FTIR detects two kinds of Bronsted sites in DHZ and a distribution of Bronsted sites in DHY. In DHZ no Leu is sites were detected by this technique. In conjunction with results obtained by high-resolution Si-29 MAS REDOR spectroscopy and reported elsewhere, a distinction is made between the Bronsted acidity of an OH group bridging a silicon with only one aluminum within its first coordination shell and at least one aluminum in its third shell (Q(1)), from an OH group on a silicon with no aluminum in its third coordination shell (FAl1). The latter OH group is more acid than the former. In DHY it is shown that the apparent activation energy of the overall n-pentane transformation increases to about 110 kJ mol(-1) as FAl1/Q(1) decreases below 1/3. Above 1/3 the apparent activation energy is about 60 kJ mol(-1) The activity of DHY and DHZ scales with FAl1. The selectivity to isomerization vs. cracking depends mainly on the zeolite structure and the partial clogging of the pores by nonframework aluminum.