The invention relates to a microelectrochemical electrode structure comprising a monolithic substrate having a front surface and a back surface facing generally away from one another, a first well extending into the substrate from the surface towards the back surface and ending in a first well bottom, and a first passage extending into the substrate from the back surface to the first well bottom. A first electrode is located wholly within the first well. A first conductor in the first passage serves for electrically communicating the first electrode to adjacent the back surface. A plurality of such electrode structures can be provided on a single substrate. The use of semiconductor processing technology allows the entire sensor to be extremely small. If desired, an integrated circuit can be provided on the back surface of the substrate for amplifying or otherwise processing signals from the first electrode. Analysis can be carried out for vapors or dissolved species (ionic or non-ionic).