A transport equation is derived which describes the behavior of the nanostructured metal oxide films in a photoelectrochem. cell. A detailed anal. of the charge compensation mechanism necessarily leads to a transport equation with characteristics similar to but logically distinct from the pure diffusion equation. The studied phenomenon was named ambipolar diffusion in the early 1950s. It takes into account the fact that the diffusion processes of ions and electrons occur at different speeds. A weak elec. field therefore couples the processes together to preserve charge neutrality. The elec. field in turn affects the transport resulting in a deviation from purely diffusive behavior. However, this was not widely recognized in the literature for nanostructured semiconductor films until very recently. A detailed anal. is presented. It is based on the assumption that the c.d. is solenoidal. Application of the ambipolar diffusion model to a photoelectrochem. cell based on a nanostructured metal oxide film leads to an addnl. term in the transport equation, rather than only a new diffusion coeff. as in earlier work. Also the boundary conditions interact closely with the equation to form a transport model.