Ultra-soft pseudopotentials plane-wave technique is used to simulate the tip-sample interaction and STM-AFM image formation on TiO2(1 1 0) 1 x 1 and 1 x 2 surfaces. It is shown that the strong tip-oxygen chemical interaction determines the main feature of the AFM image formation. By checking the effects of tip-induced surface relaxation, spin polarization, and bias voltage, we show that it should be the significant tip-induced surface relaxation on the 1 x 1 surface which leads to the too small image corrugation observed experimentally on the 1 x 1 surface. (C) 2002 Elsevier Science B.V. All rights reserved.