The electroabsorption (Stark) spectrum of the emeraldine base form of polyaniline (EB) in a polymethyl methacrylate (PMMA) matrix at room temperature is presented. For the lowest-energy (1.98 eV) band, the absolute value of the change in dipole moment on excitation (\ Delta<()over right arrow>\) is 6.1 D and the trace of the change in polarizability (Tr <(alpha )over left right arrow>) is 200 Angstrom (3). Similar values were obtained for the corresponding electronic transition of a five-ring oligomer of EB that is symmetrically terminated with phenyl rings, also in a PMMA matrix. In order to understand the origin of the substantial value of \ Delta<()over right arrow>\ measured for these nominally symmetric species, a series of calculations were performed in which the geometries of model oligomers were varied in a systematic fashion and resulting effects on the values of \ Delta<()over right arrow>\ were determined. Numerous low-energy conformers were identified by this method having substantial values of \ Delta<()over right arrow>\, suggesting that the large \ Delta<()over right arrow>\ measured experimentally results from conformational heterogeneity. Our findings suggest that Stark spectroscopy is a useful probe of structural disorder in this technologically important polymer in the solid state.