Many recent experimental studies have demonstrated that the deposition of a self-assembled monolayer (SAM) made of polar molecules on a metal surface can significantly modulate its work function and hence the barrier for hole and electron injection in optoelectronic devices. The permanent dipole moment associated with the backbone of the molecules plays a key role in defining the amplitude and direction of the work-function shift. We illustrate here via quantum-chemical calculations performed on model systems that the dipole moment of molecules is significantly reduced going from the isolated state to the SAM. Such depolarization. effects that are most often neglected thus reduce the work-function shift and have to be taken in account to control and understand charge-injection barriers in devices at a quantitative level.