Initially, we monitored the hydrocyanation of 1a under 1 H NMR control (Figure ). During the reaction progress, we observed the formation of the key intermediate 2a, whose yield initially rises to around 10% before being consumed. These findings confirm that the reaction proceeds via the initially proposed pathway, involving the isomerization of alkyne 1 to activated allene 2. Interestingly, in contrast to the phosphine-catalyzed hydrocyanation of allene 2a, the Z/E ratio of 3a remains constant during the amine-phosphine-catalyzed reaction starting from alkyne 1a (see Figure in Supporting Information), however, with the final ratios achieving similar values. This might raise the question of whether the stereoselectivity is controlled by a catalytic system or whether it is dictated by the relative energy of Z/E stereoisomers. To address this, we studied the possible isomerization of pure (Z)-3a and (E)-3a (Figure , top). The data show that Z/E mixtures with the same ratio are formed in the presence of TEA in both cases, indicating thermodynamic control of selectivity in the dual-catalytic system.