We demonstrate the use of guided wave depolarized light scattering (GWDLS) with diffraction gratings for studying the properties of soft block copolymer films in the vicinity of order-disorder transitions. The order-disorder transition temperature (T-ODT) of 500 nm thick poly(alpha-methylstyrene-block-isoprene) films is determined by this technique. Atomic force microscopy on quenched samples is used to verify our conclusion. Disordering and ordering dynamics of these films were examined in real time as the films were heated and cooled across the T-ODT. Our work opens the door for using depolarized light scattering for locating transitions between optically isotropic and optically anisotropic block copolymer phases in thin films.