Time-resolved small-angle X-ray scattering (SAXS) has been used to probe photoexcitation of the blue-light signal transduction protein Vivid (VVD). Laser excitation of sample in a continuous flow cell enables time-resolved measurement of the initial response of VVD to illumination. Good signal-to-noise is achieved without relying on multiple exposures of the same sample or limiting exposure times to prevent radiation damage. The SAXS data demonstrate that VVD dimerizes within tens of milliseconds of light-state activation. Time-resolved SAXS in a flow cell format is a general method for connecting chemical changes in photoreceptors to conformationally driven output signals.