Samples of CoP3, CoAs3 and CoP1.5As1.5 have been synthesized and their thermoelectric properties measured. All three samples show semiconducting behavior. The Seebeck coefficients of CoP3 and CoAs3 are weakly dependent on temperature and are relatively small with maximum values of about 40 and 50 μV/K, respectively. The Seebeck coefficient of the solid solution gradually decreases with increasing temperature and the values are larger than those of CoP3 and CoAs3 in the temperature range investigated, with a maximum value of about 89 μV/K near room temperature. The thermal conductivity of CoP3 and CoAs3 are higher than that of CoSb3, as can be expected from the effect of anionic size on lattice vibration. A substantial reduction in thermal conductivity was observed for the solid solution compared to the constituent binary compounds due to additional phonon scattering from lattice disorder and other possible point defects such as vacancies. Other compositions in the CoP3-xAsx system have also been synthesized and their thermoelectric properties are currently being investigated to provide essential information about lattice thermal conductivity reduction by point defect scattering and to further develop strategies for optimizing the thermoelectric properties of skutterudite materials.