Aged H-ferrierite (H-FER) samples with different contents of deposits were prepared and studied under differential catalytic conditions in a tapered element oscillating microbalance. Subsequently, these samples were examined using infrared spectroscopy to determine the nature of carbonaceous deposits and probing the type and number of accessible sites with d(3)-acetonitrile. From these results, for the first time, we have been able to calculate turnover frequencies (TOFs) for n-butene conversion and isobutene formation for both fresh and aged H-FER catalysts. It is observed that the deposition of carbonaceous species significantly lowers the number of accessible Bronsted sites. With short time on stream (TOS), cracking of the alkyl-aromatic deposits contributes to the overall isobutene production but simultaneously harms the selective catalytic action by inducing nonselective side reactions. With prolonged TOS these deposits become nonreactive and as a result the TOF drops, while isobutene is produced with high selectivity. It is demonstrated that at this stage no carbenium ions are detected, while Bronsted OH acid sites are still accessible for d3-acetonitrile on the extensively aged and highly selective H-FER. This indicates that the latter sites are responsible for the selective catalytic conversion of n-butene into isobutene.