A two-dimensional transient model of dye solar cells (DSC) describing the electrochemical reactions in the cell has been prepared. The model includes the relevant components of DSCs: the photoelectrode, the electrolyte, and the counter electrode. The solved variables are potential and the concentrations of the different ion species, which can be used to determine, e.g., the current voltage characteristics of the cell. The largest benefit of this model is its 2D features which enable the study of lateral inhomogeneity. Using the model, a new phenomenon was described: lateral current density distribution caused by a small difference in the size between photoelectrode and counter electrode, typical of laboratory test cells, causes tri-iodide to move from the edge region to the active area of the cell. This process takes a relatively long time (8 min) and can be important for performance characterization and design of DSCs.