The bulk and shear moduli of samarium phosphate glasses decrease under hydrostatic pressure: this acoustic-mode softening means that when compressed, these glasses become more compressible! The elastic behaviour also changes with temperature in an anomalous way. In contrast the structurally similar lanthanum phosphate glasses show normal elastic stiffness dependences upon temperature and pressure. Hence the source of the anomalous elastic behaviour of the samarium glasses is the samarium ion and is probably a volume-dependent valence state.