We have investigated thin GaAs layers capped with 20 A˚ Ge pertaining to the extent of the well known surface depletion layer in this semiconductor. Using the transmission line method, the effective surface potential of 0.78 V measured in the GaAs surface was reduced to 0.45 V by the epitaxially grown Ge cap layer. About 0.26 of the 0.45 V is due to the conduction‐band discontinuity at the Ge/GaAs heterointerface which leads to an actual surface potential of around 0.19 V. The same trend was also verified by photoreflectance and photoluminescence with variable excitation wavelengths. The results are encouraging for device applications despite the possibility that a considerable portion of the 20 A˚ Ge cap layer is oxidized.