We present quantitative measurements of the work function of semiconductor and metal surfaces prepared in ultrahigh vacuum (UHV) using a combination of UHV noncontact atomic force microscopy and Kelvin probe force microscopy. High energetic and lateral resolution is achieved by using the second resonance frequency of the cantilever to measure the electrostatic forces, while the first resonance frequency is used to simultaneously obtain topographic images by the frequency modulation technique. Spatially resolved work-function measurements reveal a reduced work function in the vicinity of steps on highly oriented pyrolytic graphite. On the GaAs(110) surface it could be demonstrated that defect states in the forbidden band gap cause a local pinning of the Fermi level along monolayer steps. On p-WSe2(0001) work-function variations due to the Coulomb potential of single dopant sites were resolved.