Isotope-controlled selectivity (ICS) is defined as a molecular system where one of two conceivable products resulting from a quantum mechanical tunneling (QMT) reaction forms predominantly, only depending on isotopic composition. This novel concept of controlling reactivity has been theoretically predicted but to this day, has not been demonstrated experimentally. Here we investigate the effect of isotopic substitution (hydrogen vs. deuterium) in the reactivity of singlet protoadamantylidene and provide spectroscopic evidence for the formation of different products as a result of ICS.