Disclosed is a process for producing a hydrocarbon fraction rich in components boiling in the range typical for diesel fuel comprising contacting a feedstock comprising one or more C2 to C10 alkenes with a modified zeolite catalyst having a one-dimensional micropore structure consisting of channels made from rings containing between 8 and 12 silicon/aluminum atoms at a temperature in the range 100 to 500° C. and pressure in the range 0.1 to 200 bar characterized in that the modified zeolite catalyst is one which has been prepared by treating a corresponding zeolite precursor with an alkaline solution. The alkaline solution used to treat the zeolite precursor can be for example aqueous sodium hydroxide solution. Relative to equivalent untreated zeolites the modified zeolite catalysts described show improved catalyst life and selectivity to hydrocarbons boiling in the range 250 to 350° C.