Thiocyanate ions and thiourea molecules enhance the reversibility of the reduction of copper(I) selenide Cu2Se and copper(I) sulfide Cu2S which are immobilised on a glassy carbon electrode. This behaviour is explained by the adsorption of SCN- ions and thiourea on the surface of copper(I) chalcogenites and their bridging function in an inner-sphere electron transfer reaction from the solid glassy carbon to the solid sample particles. Model calculations show that both electrocatalysts decrease the resistance for the electron transfer across the glassy carbon/Cu2Se (Cu2S) interface and simultaneously increase the rate constant of this process. This is the first example of an inner-sphere solid-to-solid electron transfer reaction.