Coke formation during naphtha pyrolysis has been studied in the presence of triethyl phosphite in a jet-mixed reactor at the temperature of 1088 K. The quantitative effect of the addition of triethylphosphite has been reported. Concentrations of iron and phosphorus in the formed coke have been analyzed by chemical methods. For fresh coupons, the asymptotic rate of fouling was found proportional to the iron content of coke. A major decomposition product of the additive has been identified by chemical analysis of an aqueous effluent and a possible mechanism is proposed for the inhibition of coke formation.