Persistence lengths of stilbene and styrene containing alternating copolymers were measured using small-angle X-ray scattering (SAXS) and size-exclusion chromatography (SEC). For SAXS measurement, a graphical approach and the Sharp and Bloomfield method of analyzing scattering data were both used to quantify the chain stiffness. For SEC measurement, the Bohdanecky linear approximation was applied to determine persistence lengths. The complementary results obtained from SAXS approaches as well as from SEC measurements characterized stilbene copolymers as a class of semirigid polymers with persistence lengths in a range of 2-6 nm. Stilbene copolymers are more rigid than their styrene analogues.