The synthesis and characterization of a seriesof 4 ‐ (4′‐ n ‐alkoxybenzylideneimino)phenyl‐mercury(II) dithizonates, [CnH2n+1OC6H4CHNC6H4Hg(Hdptc)] (H2dptc = 1,5‐diphenylthiocarbazone = dithizone; n = 1, 4, 12, 14 or 18), is described. The intermediate long‐chain organomercury(II) acetates were obtained by condensation of 4‐aminophenylmercury(II) acetate with the appropriate 4‐n‐alkoxybenzaldehydes. The thermal behaviour of the complexes was studied by polarized optical microscopy and differential scanning calorimetry but revealed no mesophases. All of the complexes, however, were reversibly photochromic (yellow ⇌ blue) upon irradiation of chloroform solutions with visible light, and the half‐lives of the metastable blue forms were substantially increased relative to that of the parent mercury(II) bis(dithizonate). The compounds were not photochromic in the solid state. Copyright © 2000 John Wiley & Sons, Ltd.