Using electrochemical STM we have studied the stability of arrays of parallel, single atom height steps on vicinal Ag(111) electrodes in electrolyte. We find that the steps for Ag(111) electrodes are unstable and form first double-steps and later multiple steps, separated by wide, flat terraces. In this paper denoted as "I: Experiment" we deal with the experimental aspects whereas theoretical aspects are discussed in the following paper "II: Theory". (C) 2004 Elsevier B.V. All rights reserved.