Clinopyroxene binary solid solutions in the systems diopside–hedenbergite, diopside–aegirine, and hedenbergite–aegirine have been synthesized. Based on data from X-ray diffraction studies, the unit-cell parameters of these solid solutions have been refined, and their concentration dependences have been established. These clinopyroxene series are near-ideal continuous solid solutions. The partition of Mg and Fe between the diopside–hedenbergite binary solid solutions and biotite of the phlogopite–annite series was studied by the method of cation exchange equilibria at 650 and 750 °C and 2 kbar. The application of KF solution as a reaction medium permitted achieving equilibrium in a considerably shorter time and from its both sides. The partition of Mg and Fe is near-ideal within the experimental error. The experimental results were compared with results of calculations based on literature data on the thermodynamic properties of pure end-members.