The writer has synthesized pure diopside and diopsides in which Na⁺¹ and Mg⁺² have been substituted for Ca⁺²; in which Al ⁺³, Fe⁺³, Cr⁺³, Mn⁺², Ni⁺², and Ti⁺⁴ have been substituted for Mg⁺²; and in which Al⁺³ and Fe⁺³ have been substituted for Si⁺⁴ . The unit-cell dimensions of these synthetic clinopyroxenes have been determined. From these data, the effects of individual (e.g., Ni⁺² for Mg⁺²) and coupled (e.g., Na⁺¹ Al⁺³ for Ca⁺²Mg⁺²) ionic substitutions on the unit-cell dimensions of pure diopside have been obtained. On the basis of the assumption that the substitution of Na⁺¹ for Ca⁺² has a negligible effect on the diopside unit-cell dimensions, the effects of individual ionic substitutions (e.g., Al⁺³ for Mg4⁺², Ti⁺⁴ for Mg⁺², Al⁺³ for Si4⁺⁴…) are assessed. Many of the findings are i n accord with the principles of crystal chemistry (e.g., increase in the c dimension with substitution of Al⁺³ or Fe⁺³ for Si⁺⁴), but others are not. In the latter case, discrepancies are probably due to errors in the data. The probable nature and magnitude of of such errors are discussed. The unit-cell dimensions of pure diopside are: a = 9.746Å, b = 8.924Å, c = 5.247Å, β= 74°5′; a sinβ(= d₁₀₀) = 9.373Å. In order of decreasing magnitude, the effects of individual ionic substitutions on these dimensions are: a sinβ decreases with substitution of Mg⁺² for Ca4⁺², Cr⁺³ for Mg⁺², and Al⁺³ for Mg⁺² and increases with substitution of Fe⁺³ for Si4⁺⁴; b decreases with substitution of Cr⁺³ for Mg⁺², Al4 ⁺³ for Mg⁺², Mn4 ⁺² for Mg⁺², Fe⁺³ for Si4⁺⁴, Ni4 ⁺² tor Mg⁺², and Al⁺³ for Si⁺⁴; c increases with substitution of Fe⁺³ for Si⁺⁴ Mg⁺² for Ca⁺², Ti⁺⁴ for Mg⁺², Al4 ⁺³ for Mg⁺², Al⁺³ for Si⁺⁴, and Fe⁺³ for Mg⁺²; β decreases with substitution of Mg⁺² for Ca⁺² and Ti⁺⁴ for Mg⁺² . Effects of other individual substitutions are slight.