A method of producing ethylene and, optionally, propylene comprising a) subjecting a feedstock selected from naphtha, C2-C4 alkanes and gas oil to steam cracking in a steam cracker having a cracker pyrolysis furnace to produce a first olefin containing stream, which is then passed into a cracked gas compression train and a caustic wash system; b) heating ethanol with heat from a steam cracker; c) passing the heated ethanol over a dehydration catalyst at a temperature between 200C to 500C preferably 250C to 450C and a pressure of from 0.02MN/m 2 to 4MN/m 2 to dehydrate the ethanol to produce a second olefin containing stream which is an ethylene rich gas product comprising acetaldehyde; d) combining the first and second olefin containing streams to give an initial product stream comprising ethylene and optionally propylene; wherein the streams are combined prior to or within the compression train, and wherein the second olefin containing stream is washed to remove the acetaldehyde content thereof prior to combination with the first olefin containing stream; and e) subjecting the initial product stream to purification comprising at least i) water content reduction by cooling the initial product stream to a temperature below 20C preferably below 15C so as to produce liquid water which can be separated from the initial liquid stream ii) hydrogen content reduction using a pressure swing absorber and/or palladium and/or platinum membrane and/or a cryogenic distillation column for separating at least some hydrogen from the initial product stream iii) reduction of content of molecules containing 4 or more carbon atoms using a distillation column and iv) ethane content reduction using a cooler for cooling the initial product stream and a distillation column arranged to give an ethylene rich- ethane poor fraction and an ethylene poor - ethane rich stream and for separating the ethylene rich -ethane poor stream from the ethylene poor ethane-rich stream. a) subjecting a feedstock selected from naphtha, C2-C4 alkanes and gas oil to steam cracking in a steam cracker having a cracker pyrolysis furnace to produce a first olefin containing stream, which is then passed into a cracked gas compression train and a caustic wash system; b) heating ethanol with heat from a steam cracker; c) passing the heated ethanol over a dehydration catalyst at a temperature between 200C to 500C preferably 250C to 450C and a pressure of from 0.02MN/m 2 to 4MN/m 2 to dehydrate the ethanol to produce a second olefin containing stream which is an ethylene rich gas product comprising acetaldehyde; d) combining the first and second olefin containing streams to give an initial product stream comprising ethylene and optionally propylene; wherein the streams are combined prior to or within the compression train, and wherein the second olefin containing stream is washed to remove the acetaldehyde content thereof prior to combination with the first olefin containing stream; and e) subjecting the initial product stream to purification comprising at least i) water content reduction by cooling the initial product stream to a temperature below 20C preferably below 15C so as to produce liquid water which can be separated from the initial liquid stream ii) hydrogen content reduction using a pressure swing absorber and/or palladium and/or platinum membrane and/or a cryogenic distillation column for separating at least some hydrogen from the initial product stream iii) reduction of content of molecules containing 4 or more carbon atoms using a distillation column and iv) ethane content reduction using a cooler for cooling the initial product stream and a distillation column arranged to give an ethylene rich- ethane poor fraction and an ethylene poor - ethane rich stream and for separating the ethylene rich -ethane poor stream from the ethylene poor ethane-rich stream. i) water content reduction by cooling the initial product stream to a temperature below 20C preferably below 15C so as to produce liquid water which can be separated from the initial liquid stream ii) hydrogen content reduction using a pressure swing absorber and/or palladium and/or platinum membrane and/or a cryogenic distillation column for separating at least some hydrogen from the initial product stream iii) reduction of content of molecules containing 4 or more carbon atoms using a distillation column and iv) ethane content reduction using a cooler for cooling the initial product stream and a distillation column arranged to give an ethylene rich- ethane poor fraction and an ethylene poor - ethane rich stream and for separating the ethylene rich -ethane poor stream from the ethylene poor ethane-rich stream.