FIELD: organic chemistry and chemical technology. SUBSTANCE: invention describes method of synthesis of substituted 4-hydroxy- -benzaldehyde of the general formula (V) 
where Z1 means linear or branched alkoxy-radical containing from 1 to 12 carbon atoms, preferably from 1 to 4 carbon atoms, for example, methoxy-, ethoxy-, propoxy-, isopropoxy-, butoxy-, isobutoxy-, sec.-butoxy-, tert.-butoxy-radicals; Z2 and Z3 are identical or different and mean hydrogen atom or one of the following groups: linear or branched alkyl-radical containing from 1 to 12 carbon atom, preferably from 1 to 4 carbon atoms, for example, methyl-, ethyl-, propyl-, isopropyl-, butyl-, isobutyl-, sec. -butyl-, tert.-butyl-radical, linear or branched alkenyl-radical containing from 2 to 12 carbon atoms, preferably from 2 to 4 carbon atoms, for example, vinyl-, allyl-radical, linear or branched alkoxy-radical containing from 1 to 12 carbon atoms, preferably from 1 to 4 carbon atoms, for example, methoxy-, ethoxy-, propoxy-, isopropoxy-, butoxy-, isobutoxy-, sec.-butoxy-, tert.-butoxy-radical, phenyl radical, halogen atom, preferably fluorine, chlorine or bromine atom. Method is characterized in that substituted phenolic compound of the general formula (I) 
as salt where Z1, Z2, Z3 have the above indicated values is subjected for carboxylation at position 6 to yield the compound of the formula (II) 
where Z1, Z2, Z3 have the above indicated values; M is metal cation of the group (1a) or quaternary ammonium cation. Then hydroxymethylation reaction at position 4 is carried out to yield the compound of the formula (III) 
where Z1, Z2, Z3, M have the above indicated values followed by oxidation of hydroxymethyl group to formyl group to yield the compound of the formula (IV) 
where Z1, Z2, Z3, M have the above indicated values. At the final stage the compound of the formula (IV) is decarboxylated to yield the end 4-hydroxybenzaldehyde of the formula (V). EFFECT: simplified technology of process. 25 cl, 1 tbl, 4 ex
