The vibrational spectrum of Brønsted acid sites in commercial zeolite catalysts as a function of framework topology and composition (Si/Al ratio) will be studied using the recently upgraded TOSCA instrument, now capable of such studies. Zeolites H-ZSM-5 (Si/Al = 25, 40, 80 and 280), HY ((30 and 80) and H-Beta (38 and 300) will be studied and the vibrational properties of the O-H Brønsted acid sites compared. The effect of topology and framework composition on the acidity (characterised by O-H vibrational properties) will be compared directly with QM/MM embedded cluster simulations to help explain such structure/composition effects, thus directly assisting in the design of improved acidic zeolite catalysts.