The optical transmittance and durability of several immersion cooling liquids that could be used for high concentration III-V photovoltaic systems are investigated. Firstly, the optical transmittance of the liquid candidates divided into three categories: synthetic oil, silicone oil and mineral oil are determined based on a double optical path-length transmittance method. The normalized photocurrent density of GaInP/GalnAs/Ge triple-junction solar cells when illuminated by the solar spectral irradiance filtered by each liquid is reported. Results show that the liquid candidates exhibit superior transmittance for the UV and visible wavelengths of interest, whereas they display some absorption bands in the 1200-1800 nm spectrum and thus the photocurrent of the bottom subcell Ge decreases. This is not an issue since the bottom subcell normally produces excess current. Then, the optical transmittance of the liquid candidates is monitored during exposure to UV light, damp heat and high-temperature accelerated aging tests. The average transmittance for the wavelengths of interest is introduced to quantify changes in the optical transmittance of immersion cooling liquids after being subjected to the accelerated aging tests. Results from the accelerated aging tests indicate that dimethyl silicone oil, white oils A/B/C and C14 n-alkane are suitable for immersion cooling of multi-junction solar cells whose average transmittance losses are less than the total degradation value of 5% allowed for CPV modules under qualification tests in the IEC62108 standard. The optimum liquid for immersion cooling multi-junction solar cells is found to be dimethyl silicone oil due to its high transmittance for the wavelengths of interest and its loss in average transmittance over each exposure period is always less than 0.5%.