We developed a sensor element for detecting formaldehyde. The sensor element was made of a porous glass impregnated with both a Schiff's reagent and an acid. The sensor element changed its color from yellow to violet after exposure to formaldehyde, and an absorption peak appeared at a wavelength of 570 or 580 nm. There was a quasi-linear relationship between the 570 and 580 nm absorbance of the sensor element after exposure to formaldehyde and a logarithm of the exposed formaldehyde concentration. Also, the sensor element worked as a reversible type, not as an accumulate type, and the absorbance at 570 or 5 80 nm of a once exposed sensor element decreased when it was exposed to a less concentrated formaldehyde atmosphere. These results indicate that our developed sensor element is reversible and suitable for the detection of a wide formaldehyde concentration range, for example from 20 ppm to 10 ppb. (C) 2007 Elsevier B.V. All rights reserved.