Among the cloud properties, cloud optical depth is particularly useful to describe the cloud, and can be determined from surface measurements of the total atmospheric transmittance in some shortwave bands. Here we use a very simple two-stream treatment of the radiative transfer to show how cloud optical depth determinations could be affected by various aerosol properties. Results show how, in connection with the fact that cloud drops and aerosol particles have different optical properties, the aerosol optical depth has an effect considerably higher than it could be expected. Likewise, the effect of the aerosol properties and the ground albedo on the cloud optical depth determination is addressed. The results are compared with simulations performed with a more rigorous (but still one-dimensional) radiative transfer code. It is found that the two-stream method considered can roughly reproduce the results obtained using more rigorous treatments. The general conclusion is that aerosol optical properties have a remarkable effect and thus they must be taken into account when retrieving cloud optical properties.