For a fundamental understanding of ultrafast dynamics in chemistry, biology and materials science it has been a long-standing dream to record a molecular movie in which both the atomic trajectories and the chemical states of every atom in matter are followed in real time(1). Free- electron lasers provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element- specific and chemical- stateselective information with femtosecond time resolution. The key challenge lies in synchronizing the free- electron lasers with separate optical lasers. We exploit the peak brilliance of the free- electron laser in Hamburg(2,3) ( FLASH) and establish X- raypulseinduced transient changes of the optical reflectivity in GaAs as a powerful tool for X- ray/ optical cross- correlation. This constitutes a breakthrough in the path towards recording a molecular movie and - equally importantly - opens up the field of femtosecond X- ray- induced dynamics, only accessible with high- brilliance X- ray free- electron lasers.