The coking of an ultrastable Y zeolite was investigated during cracking of n-octane, isooctane, and 1-octene in a fixed-bed reactor over a temperature range of 523-623 K, various residence times, and various reactant compositions. The coking tendencies were found to be in the following descending order: 1-octene > isooctane > n-octane. In all cases, the coke content increased with increasing reactant feed composition or residence time. However, the coke amount formed was not proportional to the reactant feed composition, because of a strong pseudo-zeroth-order initial coking on strong acidic sites. Coke formation during the first minute of TOS was significantly faster than at later stages, because of the very strong zeolitic acid sites, which deactivated rapidly. Differences in coke formation at different compositions started to build only later at much lower rates. Similarly, coke content increased with increasing reaction temperature at high compositions. However, at low reactant compositions, the coke content decreased with increasing temperature for isooctane and 1-octene. This was due to an increase in desorption of the olefin intermediates responsible for coke formation into the gas phase with increasing temperature.