A method of treating a substrate, which method comprises providing an electrolyte in contact with the substrate and an array of electrodes adjacent the surface and in contact with the electrolyte, and altering the potential of at least one electrode so as to generate an active redox product which modifies the substrate adjacent the least one electrode, characterized in that the electrolyte is chosen such that the active redox product is quenchable by a second redox product. The method is particularly suitable for the step-wise chemical synthesis of oligomers such as oligonucleotides bound to a surface.