Isobutane of technical quality was pyrolyzed in nitrogen as a diluent under standardized conditions (820-degrees-C, 0.4 s, N2/isobutane = 0.4 g/g) in a specially developed, vertically positioned tubular flow reactor coupled with a micro electrobalance. After cooling the dry cracked gases to temperatures thought to be representative for the innertube surface temperatures of a TLE (transfer-line exchanger), the growth of carbon-rich deposits (known as TLE fouling) was measured continuously at the surfaces of material coupons in dependence on (i) the prehistory and (ii) the sort of materials as well as (iii) the temperatures. The results show clearly that on low alloyed steels catalytic reactions can play an important role in TLE fouling if dry cracked gases are used.