Taking into account features of complexformation of molibden (VI) with azoderivates of pyroqallol we have synthesized highly sensitive reagent – bis-(2,3,4-threehydroxyphenilazo)-benzidin (R, H6R) and the given reagent has been used in direct spectrophotometric determination of molibden (VI) in clay. Complexformation of molibden (VI) with H6R is studied and it has been established, that at pH 2, λмах=490 nm intensively colored binary complex is formed. For increasing analytical parameters the influence of the third components–cation surface-active substance (CSAS) on complexformation has been studied. It is established, that during the influence of CSAS the maximum exit of different ligand complexes is observed in strongly acid medium and also decreases the limit of detection of the molibden (VI) on reaction of complexformation with Н6L-CSAS and increase constants of stability of their complexes. The influence of foreign ions and masking substances on complexformation is studied. The worked out method has been used in photometric determination of molibden (VI) in Masazyr clay