The interactions of thermally evaporated potassium atoms with poly(p-phenylene vinylene) (PPV) and its soluble derivative, poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) was studied in situ by angle-resolved X-ray photoelectron spectroscopy (XPS). The changes in the C ls core-level line shape of the polymers, the evolution of the K 2p core-level spectra, and the changes in chemical compositions at the interface with progressive deposition of the metal atoms were carefully monitored, The interactions of the K atoms with the conjugated polymer surfaces were compared with those involving metals of other work functions. Diffusion of the K atoms into the polymers and distinct charge transfer interactions were observed at the interface. The migration of bulk adsorbed oxygen to the surfaces of both polymers during the K deposition process resulted in an increase in oxygen concentration at the K/polymer interfaces. The diffusion of the adsorbed oxygen from the bulk of the conjugated polymer film played a dominant role in the interfacial reactions. The interfacial layer contains mainly oxidized metal and metal-polymer charge transfer species.