n-Hexadecane was used as a test reactant with a pulse technique over various types of commercial FCC catalysts, either equilibrated or steam-dealuminated, to study the changes in the selectivity to light C-4-C-6 alkenes over unit cell sizes ranging from 24.26 to 24.56 Angstrom at 475-525 degrees C. The activity depended on the total catalyst acidity, while the unit cell size of the zeolite component was the factor controlling the relative alkene yields. Inside a given hydrocarbon group, alkenes decreased their relative yields when conversion increased, while for a given conversion, their consumption as a function of increasing unit cell sizes was dependent on the molecular size : the higher the molecular weight, the slower the depletion. Linear alkenes showed a steady rate of change along the whole range of unit cell sizes, while branched alkenes showed a much faster consumption when unit cell sizes were under about 24.31 Angstrom. The yield of light alkenes was significantly augmented as a function of increasing temperature, linear alkenes being favored. The incidence of hydrogen-transfer reactions is discussed.