Particle-wall contact forces between a TiO2 film coated AFM tip and TiO2(110) single crystal surfaces were analyzed by means of UHV-AFM. As a reference system an octadecylphosphonic acid monolayer covered TiO2(110) surface was studied. The defect chemistry of the TiO2 substrate was modified by Ar ion bombardment, water dosing at 3 x 10(-6) Pa and an annealing step at 473 K which resulted in a varying density of Ti(III) states. The observed contact forces are correlated to the surface defect density and are discussed in terms of the change in the electronic structure and its influence on the Hamaker constant. (C) 2014 Published by Elsevier B.V.