Forming a solid solution phase with careful selection of end members and their compositions yields the opportunity to create unexpected or superior properties beyond averaging of their end member properties. To form a multi-component solid solution phase, several factors are generally considered, such as crystallographic parameters, chemical valance, and radius of cations or anions for minimum formation energy and a wide range of composition tuning. In addition to the design of chemical compositions, specific synthesis strategy is also of great importance to obtain solid solutions along with the control of homogeneous nanostructure and composition. In this talk, the rational design of solid solution phases and their synthesis strategies are introduced to improve the electrochemical properties for energy storage/conversion applications. First, ternary solid solution phases were introduced as anode materials for rechargeable alkali-ion batteries (Li+ and Na+) by either cation or anion exchange in binary metal phosphides to compensate the drawbacks of end members. Second, the critical role of the alloy composition and electrode architecture of Pt1-xRhx electrocatalysts affecting the ethanol oxidation reaction (EOR) selectivity and durability was investigated towards high-performance direct ethanol fuel cell (DEFC) applications. Details of synergistic solid solution effects in energy storage/conversion and their structure-mechanism-property relationships will be discussed in this presentation.