Formaldehyde, as its dimedone adduct (formaldemethone), has been isolated fromAscophyllum nodosum and characterised by thin-layer and overpressured layer chromatography, and from proton nuclear magnetic resonance spectroscopic and electron impact mass spectrometric data. Formaldemethone was detected in extracts of both fresh and dry alga. It is postulated that during dynamic methylation and demethylation processes in the cells, hydroxymethyl groups may be formed, which dependent on pH, are in equilibrium with either free formaldehyde or ions such as iminium, oxonium and thionium; these species will react with dimedone to form formaldemethone. The amount of formaldehyde captured by the dimedone reagent increases as the concentration of dimedone is increased, until a maximum is reached.