Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)) provided samples of varying molecular weight (Mw) in the range 20–360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performance of the devices was examined as a function of Mw. Turn-on voltages fell in the range 1.92–2.78 V, luminances varied from 231 to 5826 cd/m 2 , and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy.