The voltammetric response of a mesoporous pure silica-MCM-41 modified carbon paste electrode provides a new tool in the in situ investigation of mass transfer reactions involving copper(II) and mercury(II) species at the MCM-41\solution interface. The modified electrode has been successfully applied as a new selective sensor for these heavy metal cations in ammoniacal medium, in the presence of a large excess of alkali and alkaline earth elements.