Ambient non-contact electrostatic force microscopy (EFM) experiments in the force and force gradient modes were carried out on InAs nanoislands, MBE-grown on a GaAs surface and passivated with nitrogen. Typical EFM and Kelvin voltage images for the InAs nanoislands were obtained. Electrical force images in air can resolved single InAs nanoislands with a height down to 1 nm and a diameter of 15 run. The contrast of the EFM images was strongly influenced by the tip-individual nanoisland capacitive coupling. Local Kelvin voltage measurements revealed variations in surface contact potential, V-cp, between InAs nanoislands and the GaAs surface in the 40 mV range for the highest nanoisland. The V-cp variations are attributed to the different nature of InAs and GaAs materials. (C) 2002 Elsevier Science B.V. All rights reserved.