<p>The transfer of population between two intersecting quantum states is the most fundamental event in many dynamical processes in physics, chemistry, biology, and material science. Any two-state description of such processes requires population leaving one state to instantaneously appear in the other. We show that coupling to additional states, present in all real-world systems, can cause a measurable delay in population transfer. Using attosecond spectroscopy supported by quantum-chemical calculations, we measure a delay of 1.46 ± 0.41 fs at a charge-transfer crossing in CF₃I⁺, where an electron hole moves from the fluorine atoms to iodine. Our measurements also resolve the other fundamental quantum-dynamical processes involved in the charge-transfer reaction: a vibrational rearrangement time of 9.38 ± 0.21 fs (during which the vibrational wave packet travels to the state crossing) and a population-transfer time of 2.3–2.4 fs. Our work shows that delays in population transfer readily appear in otherwise-adiabatic reactions and predicts them to be on the order of a single-femtosecond for molecular valence-state crossings. These results have implications for many research areas, such as atomic and molecular physics, charge transfer, or light harvesting.</p>