Clays containing ruthenium purple (denoted clay/RP) were prepared and characterized by diffuse-reflectance ultraviolet-visible absorption spectroscopic and electrochemical methods. The characteristics of the clay-modified electrodes with or without RP were obtained and compared. The clay/RP electrodes displayed a remarkable ability for detection of hydrogen peroxide and ascorbate. When H2O2 or ascorbic acid was present, the current response was dramatically enhanced and tended to a limiting value, leading to a linear relationship with the bulk activity of the substrate. The rate constants (pseudo-first-order) were determined from Delta i/i(0) vs. v(-1) plots and calculated to be 530 M(-1)s(-1) (pH 3.3) for H2O2, and 150 M(-1)s(-1) (pH 4) for ascorbic acid. Flow injection analysis showed that the detection limits for both substances were about 1 ppm at pH 5. Oxygen reduction causes little interference with the detection of H2O2, and this was tentatively ascribed to unfavorable electron transfer between oxygen and clay/RP particles.