The crystal structures and metallographic features of martensites in the β phase region of Au-Cd alloys have been investigated by electron microscopy, the main emphasis being to investigate the origin of the stability of stacking variants of close packed structures which appear in martensites as compared to that of similar structures in equilibrium phases. Five types of martensite structures are found in the range of 45 at. % to 51 at. % Cd, and are designated with increasing Cd content as α′, β 2′, β 2′', γ 2′ and ζ 2′ among which the existence of two, γ 2′ and ζ 2′, had been known and been designated as β′ and β′' respectively. In addition, two structures, designated here as α 2 and α 20 are found to exist in the 45 % alloy as equilibrium phases, α′ has a disordered f.c.c. structure while β 2′ and γ 2′ are stacking variants, 3 R and 2 H respectively, of a CuAu type ordered structure. The β 2′' martensite is a distinct martensite phase but with a fine, regular lamellar mixture of β 2′ and γ 2′. The ζ 2′ martensite has an ordered hexagonal structure similar to ζ-AgZn. The composition dependence of the appearance of the crystal structures and their relative stability along with their metallographic characteristics are discussed in comparison with those in martensites in other noble metal base alloys as well as those structures found in noble metal alloys as stable states.