A room-temperature assay of formaldehyde is described. The assay uses few reagents and is colorimetric, read at a wavelength of 649 nm. Tryptophan and tryptamine were noted as interfering with the assay, probably by binding with the formaldehyde. High levels of sugar show smaller effects on final absorbance. Glyceraldehyde also reacts in the assay, but six other aldehyde compounds do not, although they do reduce the absorbance of added formaldehyde.