Some condition parameters for the one-step electrodeposition to prepare the quaternary CuIn(Se, S)(2) thin films were studied. It was found that filtering out the precipitation in the source solution after several hours from the preparation of the solution was good to obtain a given composition quaternary films. The copper to indium ratio of the films changed drastically depending on the applied potential. It became easy to control the film composition in a relatively wide range or the applied potential oy decreasing selenium concentration in the source solution.