In this study, electroabsorption (Stark) spectroscopy is used to determine the trace of the change in polarizability (tr (&UDelta;x) over bar) and the absolute value of the change in dipole moment (/Delta(μ) over right arrow/) of the electroluminescent polymer poly [2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in L solvent glass matrixes. We find a value of tr (&UDelta;α) over left right arrow of similar to2000 Angstrom(3) for the polymer and for a 9-ring substituted oligomer in both toluene and 2-methyl tetrahydrofuran matrixes at 77 K with smaller values being obtained for 3- and 5-ring unsubstituted oligomers. Although gas-phase calculations of tr /(&UDelta;α) over left right arrow/. using INDO/SCI yield values that are about a factor of 8 smaller than the experiment, excellent agreement is obtained when the effects of solid-state dielectric screening are included. Screening increases tr /(&UDelta;x) over left right arrow/ by bringing the energy gap between the 1B(u) and mA(g) states into agreement with solid-state measurements. Substantial values of /Delta(μ) over right arrow/ are observed experimentally both for the polymer and for the oligomers (6-11 D). Because in a planar (C-2h) geometry the oligomer and polymer are centrosymmetric, the observed /Delta(μ) over bar/ is an indication of disorder-induced symmetry breaking in the material. Calculations indicate that disorder in the ground-state geometry of the polymer (inner-sphere disorder) can account for nearly half of the observed /Delta(μ) over right arrow/. Disorder in the glassy environment (outer-sphere disorder) leads to a nonuniform electrostatic environment, and calculations show that this is a substantial contributor, accounting for the remainder of the observed /Delta(μ) over right arrow/.