We studied electron injection from Al and Au cathodes into the conjugated polymer poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV). When a thin CsF layer is inserted between MEH-PPV and Al, a substantial enhancement in electron injection is observed. Insertion of the same layer between MEH-PPV and Au does not have a similar effect, indicating that the enhancement mechanism is specific to CsF and Al. Thin Cs layers enhance electron injection regardless of the cathode metal. A mechanism that explains these observations is proposed.