In different parts of water-melon plants (Citrullus vulgaris L.) formaldehyde (HCHO), in dimedone adduct form (formaldemethone), and fully N-methylated substances were identified and determined by OPLC as well as HPLC, capillary GC and GC-MS methods using authentic substances. The HCHO captured originates from dynamic methylation and demethylation processes in which this simplest aliphatic aldehyde is bound in the form of highly reactive hydroxymethyl groups. Dimedone will react with the small quantity of HCHO in equilibrium with the hydroxymethyl groups and it follows from this that the rate of HCHO captured as the dimedone adduct will increase parallel to the increasing concentration of dimedone applied, as a methanolic solution, until a plateau is reached. The level of HCHO was very high in the roots of water-melon seedlings.