Optical recognition was studied with modified electrodes based on iron-rich clays. According to energy-dispersive X-ray spectroscopy (EDX). several clay minerals including montmorillonite K10 (mont. K10) contain a lot of iron. Cyclic voltammetry suggested that electrochemically active iron species exist in most iron-rich clays and are likely to reside at different sites. The associated electrochemical activity is strongly pH-dependent and photosensitive. Under UV irradiation (lambda less than or equal to 420 nm), these iron species were activated, and a pronounced photocurrent resulted. When these electrodes were flow-injected with 2-pyridylcarboxylic acids (lambda(ab, max) = 260 nm), the originally monotonic photocurrent could be modulated into a more recognizable a.c. pattern and the 260 nm optical signal became distinguishable. The photoresponse was highly reproducible, and the response time (t(90)) was less than 10 s.