Studies of the modification mechanism of An and Ti surfaces by electric fields applied with a scanning tunneling microscope (STM) are carried out at atmospheric pressure and room temperature. For pulses of 1-100 mus, nanodots can only be formed on Ti under conditions where the current shows substantial fluctuations, indicating tip-sample contact. If the field is applied gradually, Au is not affected and pits are formed on Ti. We interpret pit formation as a result of field-induced oxidation. Moreover, experiments show that the relative humidity plays an important part in nanostructure formation on Ti. Our results can be explained through a combination of direct contact and oxidation induced by electric field. (C) 2002 Elsevier Science B.V. All rights reserved.