The Coulter(R) counter is a device used to count biological cells and colloidal particles. It consists of a small aperture that separates two electrolyte solutions, one of which contains the particles to be counted. The size or the particle that can be counted is related to the size of the aperture used. We have recently shown that metal nanotubules can be electrolessly plated within the pores of microporous filtration membranes. Tubules with inside diameters that approach molecular dimensions (<2 nm) have been prepared. Because the inside diameter of these nanotubules can be of molecular dimensions, it seemed possible that a Coulter(R)-type device based on the nanotubule membranes might be able to count molecules. If so, this device might provide a new and general approach for determining trace concentrations of molecules in solution. A prototype device was constructed and used to determine the concentrations of the cations Ru(bpy)(3)(2+) (bpy = 2,2-bipyridine) and methylviologen in aqueous solution. Concentrations of Ru(bpy)(3)(2+) as low as 10(-9) M could be detected.