Patterned cell cultures obtained by microcontact printing have been modified in situ by a microelectrochemical technique. It relies on lifting cell-repellent properties of oligo(ethylene glycol)-terminated self-assembled monolayers (SAMS) by Br-2, which is produced locally by an ultramicroelectrode of a scanning electrochemical microscope (SECM). After Br-2 treatment the SAM shows increased permeability and terminal hydrophobicity as characterized by SECM approach curves and contact angle measurements, respectively. Polarization-modulation Fourier transform infrared reflection-absorption spectroscopic (PM FTIRRAS) studies on macroscopic samples show that the Br-2 treatment removes the oligo(ethelyene glycol) part of the monolayer within a second time scale while the alkyl part of the SAM degrades with a much slower rate. The lateral extension of the modification can be limited because heterogeneous electron transfer from the gold support destroys part of the electrogenerated Br-2 once the monolayer is locally damaged in a SECM feedback configuration. This effect has been reproduced and analyzed by exposing SAM-modified samples to Br-2 in the galvanic cell Au vertical bar SAM vertical bar 5 mu M Br-2 + 0.1 M Na2SO4 parallel to 10 mu M KBr + 0. 1 M Na2SO4 vertical bar Au followed by an PM FTIRRAS characterization of the changes in the monolayer system.