A new optical technique for characterizing the grain structure of ordered block copolymer thin films has been developed. The technique, which we refer to as guided wave depolarized light scattering (GWDLS), is an adaptation of previous work wherein polarized light was used to characterize the grain structure in bulk block copolymer samples [Newstein et al. Macromolecules 1998, 31, 64]. Spin-casting and vacuum-annealing were used to prepare thin films of an ordered poly(a-methylstyrene-block-isoprene) copolymer on a flat, fused silica substrate. The ordered phase consisted of poly((x-methylstyrene) cylinders in a polyisoprene matrix. A plane-polarized laser beam was coupled into and out of a transverse magnetic (TM) mode of the film, which acts as a planar waveguide. The angle of incidence is adjusted to optimize the coupling efficiency into a particular mode of the waveguide. The path length traveled by the guided wave between the input and output coupling points was approximately I cm. The polarization of some of the incident light changes due to encounters with randomly oriented, optically anisotropic grains. This results in the coupling of light into propagating transverse electric (TE) modes in the sample. We show that the TE beam intensity of annealed samples with well-developed grain structure is significantly larger than that obtained from unannealed samples with poorly developed grain structure. The GWDLS results are consistent with atomic force microscopy results obtained from the annealed and unannealed films.