The crystal and calculated band structures and electric, magnetic, and optical properties of two cation radical salts of ethylenedioxy-ethylenedithio-tetrathiafulvalene (EDOEDT-TTF: EOET): alpha'-(EOET)(2)AuBr2 and beta''-(EOET)(2)-AuBr2, are described. The alpha' salt is a Mott insulator with an EOET dimer as a unit with an S = 1/2 spin. The dimers form a two-dimensional square lattice layer. The spin dynamics follow the quadratic layer antiferromagnetic model above 60 K and then pass through a three-dimensional antiferromagnetic transition at around 9 K. The beta'' salt shows a metallic behavior down to low temperatures.