Proton transfer is a fundamentally important chemical reaction in chemistry and biology. It can be one of nature’s fastest reactions, occurring within the time span of a molecular vibrational period (<10 femtoseconds; 1 femtosecond = 1 millionth-of-a-billionth of a second). Light can trigger proton transfer in molecular excited electronic states, which is a critical step in many applications such as optical data storage and biological imaging. However, we do not yet understand how excited state proton transfer can occur so rapidly. To directly determine these dynamics, we need new experimental methods with sufficient time resolution and sensitivity to local electronic structure and nuclear geometries. This proposal develops a new technique to measure excited state proton transfer on its natural timescale using broadband light pulses in the UV/visible and soft X-ray regimes. This proposal will produce a real-time, molecular-level description of the excited state proton transfer mechanism in a prototypical molecule from the perspective of individual carbon atoms.