The effects of crystal field stabilization of Fe2+, Co2+, and Ni2+ on isothermal bulk moduli of divalent monoxides are investigated on a simple crystal field theory. The calculated bulk moduli of high-spin FeO, CoO, and NiO agree well with experimental data. The molar volumes and bulk moduli of low-spin MnO, FeO, and NiO are also calculated. The ionic radii of low-spin Mn2+, Fe2+, and Ni2+, claculated from the molar volumes, agree with Shannon and Prewitt's (1969) prediction. The bulk modulus of low-spin FeO is calculated to be 2.23--2.83 Mbar at zero pressure. The spin-pairing transition gives a lower bulk sound velocity density ratio than polymorphic transitions of oxides and silicates. This indicates that the spin-pairing transition in the lower mantle may favor a mean atomic weight of the lower mantle that is smaller than previous estimates where the effect of spin-pairing transitions were not taken into account.