Formaldehyde has been detected in drinking water supplies across the globe and on board NASA spacecraft. A rapid, simple, microgravity-compatible technique for measuring this contaminant in water supplies using colorimetric-solid phase extraction (C-SPE) is described. This method involves collecting a water sample into a syringe by passage through a cartridge that contains sodium hydroxide, to adjust pH, and Purpald, which is a well-established colorimetric reagent for aldehydes. After completing the reaction in the syringe by agitating for 2 min on a shaker at 400 rpm, the 1.0-mL alkaline sample is passed through an extraction disk that retains the purple product. The amount of concentrated product is then measured on-disk using diffuse reflectance spectroscopy, and compared to a calibration plot generated from Kubelka-Munk transformations of the reflectance data at 700 nm to determine the formaldehyde concentration. This method is capable of determining formaldehyde concentrations from 0.08 to 20 ppm with a total work-up time of less than 3 min using only 1-mL samples.