The quantum efficiencies of photoluminescence (PL) and electro-luminescence (EL) of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV) were significantly increased by heat treatments under vacuum with further removing the undissolved portion. The UV-vis absorption was found to decrease with heating time, while PL intensity increased. The maximum PL quantum yield was 6.5 times that of the untreated MEH-PPV, which was attributed to the reduction of chain aggregations and the interruption of conjugation length. The maximum EL quantum yield of their prepared ITO/PANI/MEH-PPV/Ca/AL light emitting diodes (PLED) was 46 (at 3 V) times that of the untreated sample. A typical turn-on voltage of 2.5 V for MEH-PPV PLED was able to decrease to 1 V after heat treatments, which was believed to result from the decrease of cis linkages in the polymer chains as revealed by the H-1 NMR spectroscopy. (c) 2005 Wiley Periodicals, Inc.