Ultrapure preparation conditions equivalent to ultrahigh vacuum conditions of 5 × 10-10 mbar allow the reproducible deposition of magnetic films in the monolayer (ML) range from aqueous electrolytic solutions. We used this cost and time efficient technique to prepare ultrathin CO films on Cu(001) substrates. A unique feature of electrodeposition is its ability to dissolve previously deposited layers. The thickness of deposited films can be adjusted in situ by dissolution without damaging the rest of the film. This might he useful in precisely tailoring film properties, for example in magnetic multilayers. In situ magneto-optical Kerr effect (MOKE) measurements are a simple and efficient tool for such a purpose and provide a thickness resolution of better than 0.2 ML. The magnetic properties and anisotropies of the electrodeposited films as derived from our in situ MOKE measurements are mainly determined by their epitaxial growth on Cu(001).