Olefin feeds with high olefin content and/or containing a substance that generates water when contacting the catalyst, are oligomerised over solid phosphoric acid catalyst in tubular reactors by introducing the olefin feed into the reactor and maintaining the reacting mixture under conditions whereby the peak temperature is controlled to be below 265° C. and preferably a single liquid or dense phase is maintained and the average temperature throughout the reactor is maintained in the range 190° C. to 260° C.