At the example of cross-sectionally prepared p-p(+)-silicon epilayers on silicon, this work studies the frequency dependence of the contact potential difference detected in Kelvin probe force microscopy. The experimental setup employs a single-path off-resonance technique, which has been modified in order to enlarge the frequency range to 1-330 kHz. It works with stiff cantilevers at approximately 340 kHz resonance frequency and electrical signal amplitudes below 20 pin. The setup utilizes multiple-channel digital lock-in amplification and a dedicated background correction, whose principle and experimental realization is described in detail and whose effect on the contact potential difference results is studied exemplarily. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3039682]