Colourless silver (I)-gelatin complex is quantitatively reduced by formaldehyde to yellov/ silver sol in the presence of 10 U NaQH at room temperature. The determination of formaldehyde at microgram level is possible at 440 nm in the presence of methanol, ethanol, acetone and also in the presence of various reducing agents. The molar absorptivity of the 4 -1 -1 solution is 2.16 × 10 lit mol cm with a relative standard deviation of 0.105%, a confidence limit (for 10 determinations)of 0.9 ± 0.0006 and Sandell sensitivity 1.38 × 10 μg cm−2.