This note analyzes primary products (reaction products with the same carbon atom numbers as in feeds) produced in cracking reactions of several C-7-C-9 isoalkanes over zeolite-based catalysts under very mild conditions, at 150-250 degrees C and at high [feed]:[catalyst] ratios, and discusses formation mechanisms of these products. Primary products from methyl-substituted alkanes at low temperatures are predominantly isoolefins with the same skeleton structures as in feed alkanes. A chemical mechanism which was earlier proposed to describe formation of light cracked products from isoalkanes [J. Catal. 163, 50-62 (1996)] is well suited to describe the formation of the primary cracked products as well. This mechanism includes a reaction between an isoalkane and the Bronsted center on the catalyst surface with the formation of a transient hydrosiloxonium ion -->Si-O+(H)-C<--.The ion undergoes a hydrogen atom abstraction from the C-H bond in the beta-position to O+ with the formation of an olefin molecule which, in turn, rapidly isomerizes via the carbocationic mechanism,