The addition of some methanol to petroleum light naphtha used as feed in the Thermal Catalytic/Steam Cracking(TCSC) process significantly increases the product yield of C-2-C-4 olefins, particularly that of ethylene + propylene. However, over 20-25 wt% of methanol content in the naphtha feed, the beneficial effect is attenuated. At relatively high values of contact time, the (Zn-Pd) co-catalyst of the hybrid catalyst exerts noticeably its coke cleaning effect on the zeolite acid sites, particularly when methanol is present in the feed. The product weight ratio (propylene/ethylene) is not affected by such "moderate" addition of methanol and remains higher than 1.3.