Intersystem crossing in thermally activated delayed fluorescence (TADF) materials is an important process that controls the rate at which singlet states convert to triplets; however, measuring this directly in TADF materials is difficult. TADF is a significant emerging technology that enables the harvesting of triplets aswell as singlet excited states for emission in organic light emitting diodes.We have observed the picosecond time-resolved photoluminescence of a highly luminescent, neutral copper(I) complex in the solid state that shows TADF. The time constant of intersystem crossing is measured to be 27 ps. Subsequent overall reverse intersystem crossing is slow, leading to population equilibration and TADF with an average lifetime of 11.5 μs. These first measurements of intersystemcrossing in the solid state in this class ofmononuclear copper(I) complexes give a better understanding of the excited-state processes and mechanisms that ensure efficient TADF.