Zintl phases are ideal candidates for efficient thermoelectric materials, because they are typically small-bandgap semiconductors with complex structures. Furthermore, such phases allow fine adjustment of dopant concentration without disrupting electronic mobility, which is essential for optimizing thermoelectric material efficiency. The tunability of Zintl phases is demonstrated with the series CaxYb1-xZn2Sb2 (0 <= x <= 1). Measurements of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity (in the 300-800 K temperature range) show the compounds to behave as heavily doped semiconductors, with transport properties that can be systematically regulated by varying x. Within this series, x = 0 is the most metallic (lowest electrical resistivity, lowest Seebeck coefficient, and highest carrier concentration), and x = 1 is the most semiconducting (highest electrical resistivity, highest Seebeck coefficient, and lowest carrier concentration), while the mobility is largely independent of x. In addition, the structural disorder generated by the incorporation of multiple cations lowers the overall thermal conductivity significantly at intermediate compositions, increasing the thermoelectric figure of merit, zT Thus, both zT and the thermoelectric compatibility factor (like zT, a composite function of the transport properties) can be finely tuned to allow optimization of efficiency in a thermoelectric device.