In this paper we report about coke formation during catalytic cracking of n-hexane and 1-hexene over acidic ultrastable Y zeolite in a fixed-bed reactor. The study is based on a classification between coke precursors, whose amount is estimated through volatilisation by nitrogen purging, and coke, large formed coke molecules that remain during nitrogen purging even at 873 K and whose amount is estimated by oxidation at that temperature. The role of coke precursors on the coking process and catalytic cracking reaction is discussed as well as differences in coking tendencies between the two reactants. The results show that the amount of coke precursors and/or coke may decrease with reaction temperature resulting into a reversal of the coking dependence upon temperature.