Tin (Sn) films have been prepared on a molybdenum substrate covered with copper (Mo/Cu) by the electrodeposition method from the electrolyte solutions of tin sulfate (SnSO4) and trisodium citrate (C6H5Na3O7). The formation mechanism and microstructures of the films were investigated by cyclic voltammetry (CV), chronoamperometry (CA), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The CV and CA measurements show the electrodeposition is controlled by the diffusion process of tin ions. By means of Scharifker-Hills model and microstructural analysis, the Sn nucleation mechanism is clarified to be an instantaneous nucleation mechanism. The influence of deposition time, solution concentration and the potential on microstructures of Sn films has been systematically investigated and the optimal growth parameters for preparing dense Sn films are obtained. (C) 2015 The Electrochemical Society.