Ultrasonic and piezoelectric studies of two-dimensional layered crystalline structures of MM 1 P 2 X 6 (M,M 1 =Cu, In, Sn, Cr; X=S, Se) are presented in this contribution. It was shown, that such materials: CuInP 2 S 6 , CuCrP 2 S 6 and CuInP 2 Se 6 undergo phase transitions (PT) and exhibit piezoelectric effect in low temperature phases. Anomalies of ultrasonic velocity and attenuation at phase transitions have been observed in these materials. Measurements of temperature dependencies of ultrasonic velocity and attenuation revealed the first order phase transition at T c ≈312K in CuInP 2 S 6 ; two successive phase transitions at T c1 ≈180K and T c2 ≈145K in CuCrP 2 S 6 ; and two phase transitions at T c1 ≈235K and T c2 ≈228K in CuInP 2 Se 6 .