Abstract Transition metal phosphides are promising, selective, and air‐stable nanocatalysts for hydrogenation reactions. However, they often require fairly high temperatures and H 2 pressures to provide quantitative conversions. This work reports the positive effect of phosphine additives on the activity of cobalt phosphide nano‐urchins for the semi‐hydrogenation of phenylacetylene. While the nanocatalyst's activity was low under mild conditions (7 bar of H 2 , 100 °C), the addition of a catalytic amount of phosphine remarkably increased the conversion, e. g ., from 13 % to 98 % in the case of P n Bu 3 . The heterogeneous nature of the catalyst was confirmed by negative supernatant activity tests. The catalyst integrity was carefully verified by post‐mortem analyses (TEM, XPS, and liquid 31 P NMR). A stereo‐electronic map was proposed to rationalize the activity enhancement provided over a selection of nine phosphines: the strongest effect was observed for low to moderately hindered phosphines, associated with strong electron donor abilities. A threshold in phosphine stoichiometry was revealed for the enhancement of activity to occur, which was related to the ratio of phosphine to surface cobalt atoms.