We study thin films of semi-crystalline regioregular poly(3-hexylthiophene) (RR-P3HT) and amorphous regiorandom P3HT (RRa-P3HT) in blends with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Ultraviolet and (angle-resolved) X-ray photoelectron spectroscopy techniques together with absorption, photoluminescence and optical microscopy were used to measure electronic energy levels, vertical chemical compositions and optical properties of these films. We find that ordering the P3HT chains raises the highest occupied molecular orbital (HOMO) energy level of P3HT thin films (reducing ionization potential) and that the ordering of P3HT chains occurs preferentially at the film-air interface in RR-P3HT:PCBM thin films. This leads to a vertical phase separation between P3HT and PCBM molecules, which may be undesirable for conventional P3HT:PCBM solar cells. © 2011 American Chemical Society.