Compressibility measurements up to 29.8 kbar (10 kbar=1 GPa) are presented for a number of superconducting ternary molybdenum sulphides and selenides as well as the binary compound Mo₆Se₈. The percent volume change ( Delta V/V₀) exhibits a strong nonlinear dependence on applied pressure. Values of the bulk modulus (B₀) and its first pressure derivative (B₀') are obtained by fitting these data to the Murnaghan equation of state. This analysis reveals a small bulk modulus for each material, with values comparable to those found in simple s-p elements such as indium. The data are also utilised to convert previously measured pressure effects on the superconducting transition temperature into the more physically meaningful volume dependence of T_c.