Carbon-carbon bond cleavage and dehydrogenation are observed following low-pressure activation of isobutane over HZSM-5. Temperature-dependent rate parameters were determined by flexible least squares modelling of the propene and isobutene formation data. Apparent activation energies are 73.9 +/- A 2.4 kJ mol(-1) (C-C bond cleavage) and 62.8 +/- A 1.0 kJ mol(-1) (dehydrogenation). Such low energy barriers under molecular flow conditions suggest activation steps that are obscured by higher-pressure catalysis.