Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time study of the hydrogen elimination reaction of 1,4- cyclohexadiene. The experimental observation of the ultrafast stepwise H- elimination elucidates the reaction dynamics and mechanism. With density- functional theory (ground-state) calculations, the nature of the reaction (multiple) pathways is examined. With the help of recent conical-intersection calculations, the excited-state and ground-state pathways are correlated. From these experimental and theoretical results we provide a unifying picture of the thermochemistry, photochemistry and the stereochemistry observed in the condensed phase.