Elastic and mechanical properties such as Young's modulus E, Poisson's ratio v, Debye temperature theta (D), Vickers hardness H-v, fracture toughness K-c, and fracture surface energies gamma (f) of yCuO(x). (100-y)P2O5 glasses (y = 45, 50, 55) with different copper valence states, i.e., R(Cu+) = Cu+/(Cu+ + Cu2+), at room temperature (humidity 64%) have been examined. The following features have been found: (1) the glass transition temperature (218-434 degreesC), H-v (2.7-4.4 GPa), E (50.6-78.2 GPa), and theta (D) (358-434 K) decrease largely with increasing R(Cu+); (2) the mean atomic volume, K-c (0.56-1.14 MPa.m(1/2)), and gamma (f) (1.9-11.2 J.m(-2)) tend to increase with increasing R(Cu+); (3) 50CuO(x). 50P(2)O(5) glasses with R(Cu+) = 0.42 and 0.55 have a high resistance against crack formation in Vickers indentation tests and no crack is observed in the 45CuO(x). 55P(2)O(5) glass with R(Cu+) = 0.57 under an applied load of about 98 N. The results demonstrate that elastic and mechanical properties of yCuO(x). (100-y)P2O5 glasses depend strongly on the copper valence state and the CuOx/P2O5 ratio. The unusal mechanical and elastic properties of copper phosphate glasses are well explained qualitatively by considering unique oxygen coordination and bonding states of Cu+ ions, i.e., lower coordination number and more covalent bonding compared with Cu2+ ions.