The conversion and run length for oligomerisation of olefins over a molecular sieve catalyst in a tubular reactor is improved by controlling the peak temperature to not exceed 50 degrees C. above the temperature of the temperature control fluid exiting the shell side outlet of the reactor. A tubular reactor containing molecular sieve catalyst is provided with a multipoint thermocouple in at least one tube, and optionally with a bottom design adapted for fast unloading of the molecular sieve catalyst from the tubular reactor.