We present a new electrochemical cell design, which enables in situ tunneling microscopy (STM), cyclic voltammetry, and optical measurements in a single setup and at the same surface area of the substrate. The design is compatible with the Nanoscope III system and allows the deposition and investigation of metal films down to the submonolayer range. We show in situ STM of the Cu(001) surface, cyclic voltammetry of the deposition of ultrathin Co films on Cu(001) from an aqueous Na₂SO₄/CoSO₄ solution and in situ magnetization measurements by magneto-optical Kerr effect on these Co films in the monolayer range.