In the present paper we report local work function measurements on the Cu(111)-Au and Pt(111)-Ag surfaces using scanning tunneling microscopy (STM).By measuring the response of tunneling current to the variation of the tunneling gap distance a work function image can be obtained simultaneously with a topographic STM image. By means of this technique,we are able to detect the difference between the local work functions of the overlayer and the substrate in both Cu(111)-Au and Pt(111)-Ag cases.Our results show that the local work function of the Au overlayer is between those of Au(111) and Cu(111) surfaces,supporting the results obtained with other techniques. In the case of the Pt(111)-Ag surface the local work function depends on the local thickness of the Ag overlayer.