The formation and the dissociation of metal hydrides are key steps within the hydrogen evolution reaction (HER) pathway for photochemical water splitting, but also impacts a wide range of other catalytic, industrial, and biochemical reactions. Herein, we describe our recent work studying HER at the interface between two immiscible electrolyte solutions (ITIES), between water|1,2-dichloroethane. This is a unique platform for evaluating the kinetics/thermodynamics for metallocene hydride formation using decamethylruthenocene. In this approach, an aqueous acid serves as the proton source and is pumped across the ITIES via an externally applied potential or the use of a phase transfer catalyst. Simulated curves developed using COMSOL Multiphysics software and compared to experimental ones, indicate a modified EC′ (electrochemical–chemical) mechanism for the decamethylruthenocene hydride formation. In the proposed pathway, decamethylruthenocene hydride is metastable in 1,2-dichloroethane and persists on the time scale of the recorded cyclic voltammograms long enough to transfer to the aqueous phase where it quickly dissociates. This is evidenced through an asymmetric, ion transfer wave observed experimentally and concluded to be hydride transfer. Shake-flask experiments with head space gas sampling demonstrated that hydrogen production was observed only when the biphasic system was positively polarized, to favor proton transfer, and decamethylruthenocene was photoactivated. This approach, combining electrochemical, simulation, and chromatographic methods, brings new insight into the factors that underlie the mechanism and rates of hydride formation/dissociation at soft interfaces.