Timing results are presented obtained with a time definition system using photomultiplier dynode pulses without any further shaping. The dynode pulses are fed into a zero crossover time discriminator with upper and lower level energy selectors. Timing measurements were performed with the XP2020(Q) photomultiplier and several scintillators, being small NE111 cylinders, two different sizes of Pilot U conicals and BaF 2 cylinders. The best time resolution of 100 ± 2 ps fwhm per detector pair and 60Co has been obtained with the NE111 scintillators. Several positron lifetime spectrometers have been built with this timing system. The best time resolution measured with 22Na was 145 ps fwhm for the NE111 scintillators. With BaF 2 scintillators, a resolution of 162 ps fwhm could be obtained. A detailed description of the adjusting procedure is given which ensures reproducibility of the resolutions within a few ps.