A process (or steam cracking a hydrocarbon feedstock containing olefins to provide increased light olefins in the steam cracked effluent, the process comprising passing a first hydrocarbon feedstock containing one or more olefins through a reactor containing a crystalline silicate to produce an intermediate effluent with an olefin content of lower molecular weight than that of the feedstock, fractionating the intermediate effluent to provide a lower carbon fraction and a higher carbon fraction, and passing the higher carbon fraction, as a second hydrocarbon feedstock, through a stream cracker to produce a steam cracked effluent.