A molecular valve, consisting of a poly(acrylic acid) gel-coated Au mesh and a Au mesh working electrode, allowed electrochemically control of a concentration where the valve opens. At open circuit and when +0.9 V vs. Ag/AgCl was applied to the Au mesh electrode, the valve opened at concentration higher than 3-4 x 10(-4) M. At -0.7 V it increased to 9 x 10(-4) M. On the other hand, it remained virtually constant for Ca2+ at those potentials.