Room temperature equations of state (EOSs) have been determined for synthetic and natural orthopyroxenes on and near the (Mg,Fe)SiO3 join by single-crystal X-ray diffraction to a maximum pressure of 11 GPa. The values of the room pressure bulk moduli, K0,T, and its pressure derivative, K0', are shown to be independent of the form of the equation of state used to fit the data. Variation in the Fe2+ content of the orthopyroxene has very little effect on either K0,T or K0', with the EOS parameters of synthetic FeSiO3 orthoferrosilite being K0,T=95.1 (3.0) GPa, and K0'=10.6 (1.6), both within one combined experimental estimated standard deviation of those previously determined for synthetic MgSiO3 orthoenstatite. However, substitution of even very small amounts of Ca2+ (and Al3+) to the orthopyroxene structure causes an increase in K0,T by some 14%. The density difference between the synthetic MgSiO3 and natural orthoenstatites (calculated from the equations of state at 10 GPa) is of the order of 3.4%, with a corresponding unit-cell volume difference of the order of ~0.3%. It is concluded that pure MgSiO3 orthoenstatite is not a good representation of the pyroxene component of the Earth's upper mantle.¿ 1997 American Geophysical Union