This invention provides a process for the production of diesel boiling range hydrocarbons, the process including at least the steps of obtaining an olefinic feed stream from one or more hydrocarbon producing processes wherein the olefinic feed stream contains branched short chain olefins having a chain length of from three to eight carbon atoms, and contacting the feed stream with a shape selective medium pore acid zeolite catalyst in a pressurised reactor at elevated temperature so as to convert said short chain olefins to higher hydrocarbons. The invention also provides an apparatus for carrying out the process and recovering the catalyst for reuse.