We present new experimental evidence that further confirms that a combination of electrochemical reactions and diffusion-convection (ERDC) mass transfer accounts for the potential oscillations that appear under conditions of transport limited current. A typical example is given for the reduction of Fe(CN)^ in alkaline solution accompanying periodic hydrogen evolution. No potential oscillations occur by simply replacing the hydrogen evolution with I03" reduction as the second current carrier. That replacement removes only the convection mass transfer induced by the hydrogen evolution, and retains the negative differential resistance (NDR) from the Frumkin repulsive effect. The key role of hydrogen evolution is thus to restore the Fe(CN)e surface concentration after its depleting to zero by diffusion-limited reduction, rather than purely a second current carrier. Therefore, the other mechanism, which emphasizes the NDR from the Frumkin interaction due to electrostatic repulsion, is excluded because it does not have a direct connection with the oscillations. Moreover, a crossing cycle in cyclic voltammograms is a more convincible criterion for this category of electrochemical oscillators than the negative impedance