We have synthesized and investigated the electronic properties of poly[2-(2′-ethylhexyloxy)-5-phenyl-1,4-phenylenevinylene], 12, and its methylsulfonyl derivative poly[2-(2′-ethylhexyloxy)-5-(4″-methylsulfonylphenyl)-1,4- phenylenevinylene], 13. Polymer 13 contains a second-order nonlinear optic chromophore which is in conjugation with the polymer backbone. Polymers 12 and 13 have been used as the electroluminescent (EL) layers in single layer light-emitting diodes (LEDs) (ITO/polymer/Al). We have found that when 12 was used, the LEDs had an external EL quantum efficiency of 0.01%. Attachment of the methylsulfonyl moiety to 12 to give 13 in an attempt to increase the electron affinity of the parent polymer was found to make no difference to the efficiency of the single layer EL devices and appeared qualitatively to reduce their lifetimes. © 1999 American Chemical Society.