The compositional variation of the elastic coefficients C-11 and C33 for TlGa(S1-xSex) and TlInxGa1-xS2 layer mixed crystals are discussed using Brillouin scattering measurements. Clear anomalies are observed for the longitudinal phonons propagating parallel to the (00 1) plane (elastic coefficient C-11) at x(c) congruent-to 0.8 and x(c) congruent-to 0.3 in TlInxGa1-xS2 and TlGa(S1-xSex)2, respectively. These anomalies are explained by the formation of infinite clusters. A considerable difference in the rate of change of the elastic coefficient C33 with the composition for the mixed crystals TlGa(S1-xSex)2 and TlInxGa1-xS2 is observed. The temperature variation of C33 for a TlIn0.95Ga0.05S2 mixed crystals is also given and the results are discussed.