Solar energy has been gaining significant attention as a crucial renewable energy source worldwide. Solar energy is commonly harvested using surface absorbers in traditional solar thermal systems. In the surface solar absorbers, a noticeable temperature drop could form between the absorber and heat transfer fluid, which leads to a reduction in the overall thermal efficiency. Recently, much attention has been directed to using volumetric absorbing fluid, which can directly convert sunlight into thermal energy of the fluid. This chapter discusses synthesis and characterization of several types of volumetric solar absorbing fluids and their application in two-phase loop thermosyphon. It starts with an introduction on volumetric solar absorbing fluids, followed by discussion of synthesis and property characterization of those fluids. Recent experimental results on promising solar absorbing fluids are summarized, but emphasis is placed on the stability of the black dye aqueous fluids undergoing two phase boiling and condensation processes.