We report on high-resolution potential measurements across complete metal/organic molecular semiconductor/metal structures using Kelvin probe force microscopy in inert atmosphere. It is found that the potential distribution at the metal/organic interfaces is in agreement with an interfacial abrupt potential changes and the work function of the different metals. The potential distribution across the organic layer strongly depends on its purification. In pure Alq 3 the potential profile is flat, while in nonpurified layers there is substantial potential bending probably due to the presence of deep traps. The effect of the measuring tip is calculated and discussed.