Kelvin probe force microscopy in ultrahigh vacuum is a sensitive tool to determine the absolute work function of sample surfaces. For the detection of the electrostatic force, an a.c.-voltage with a frequency omega is applied between tip and sample. Two different methods can be employed for the measurement: the frequency modulation (FM) mode or amplitude modulation (AM) mode detection. In FM-mode the oscillation of the frequency shift at omega is measured, which is proportional to the gradient of the electrostatic force. In AM-mode the amplitude of the cantilever oscillation at omega is measured which is proportional to the electrostatic force itself. A detailed comparison between AM and FM detection modes is presented, using measurements of gold islands on highly oriented pyrolytic graphite. The effect of tip geometry was studied for both detection modes. In the FM-mode mainly the tip apex contributes to the measured signal due to the detection of the force gradient, however, a.c.-voltages of around 2 V are necessary to perform stable measurements. A much stronger influence of the tip shape was determined for AM detection, reducing the contrast in the measured contact potential difference images. Nevertheless, the main advantage of the AM-mode is the possibility to use a.c.-voltages as low as 100 mV, which allows absolute work function measurements also on semiconductor surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.