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 zeolite catalyst having partially neutralised acidity and a one-dimensional or two dimensional micropore structure consisting of channels made from rings containing between 10 and 12 silicon/aluminium atoms at a temperature in the range 373 to 773 K and pressure in the range 0.1 to 200 bar characterised in that the partially neutralized zeolite catalyst contains both protons and basic cations. The basic cations are preferably selected from the group comprising Group IA and IIA cations (preferably sodium, potassium, caesium or mixtures thereof). Relative to their equivalent fully protonic forms the partially neutralized zeolite catalysts described show improved catalyst life and selectivity to hydrocarbons.