Quantum efficiency measurements under forward voltage bias conditions often show a large response in the wavelength region from 350 to 500 nm. It is seen that the light induced changes do not only cause a higher current in the illuminated case than in the dark case at forward bias (the situation which is explained in earlier work), but also often on the contrary a lower current at moderate forward bias (the current direction can be deduced from the phase shift between input signal and response signal). This second situation can be studied in detail by comparing the I-V curves in dark to the ones illuminated with the monochromatic light under consideration. Also the dependence of the response on the illumination intensity can be clearly shown in these measurements. A model for this situation is built up, making use of traps at the CdTe/CdS interface and extending a previously presented model. This model shows that the quantum efficiency measurement is a sensitive method to determine the presence of interface states. (C) 2003 Elsevier Science B.V. All rights reserved.