The effectiveness of benzyl diethyl phosphite as a coke inhibitor during naphtha pyrolysis was investigated in a tubular reactor at 1073 K and atmospheric pressure. Significant reduction in the coking rate was achieved by adding benzyl diethyl phosphite to the feed. The effect of toluene, methylcyclohexane, 1-octene and carbon disulfide on the rate of coke formation was also investigated. Toluene enhanced the rate of coke formation significantly, whereas with methylcyclohexane and 1-octene the increase was less appreciable. Carbon disulphide decreased the amount of coke deposited initially, but with increasing run time the asymptotic coking rates were found to be higher than for straight-run naphtha. The yields of propylene, 1,3-butadiene and butenes increased on addition of carbon disulphide.