In the current work, the effect of four different fluids ethylene glycol, ethanol, methanol, and acetone as well as their aqua solutions on the performance of the thermosyphon evacuated tube heat pipe solar collector is investigated. Water is used as a heat transfer fluid with a volume flow rate of 1.95 l/min. Five modules each of three evacuated tube heat pipe solar collectors are used. The thermosyphon heat pipes are manufactured and charged with each working fluid and its aqua solution of volume concentration of 0, 0.25, 0.5, 0.75 and 1.0 respectively. The performance of the systems is illustrated by the solar collector efficiency and the overall system efficiency. The results showed that pure water gives higher solar collector efficiencies than that of ethylene glycol, ethanol, methanol, and their solutions while acetone gives higher solar collector efficiency than the pure water. Correlation equations for solar collector efficiency in terms of (Tm-T)/G and concentration are deduced. The current results are validated with the previously experimental published correlations. η 𝑠 System efficiency THPECT Thermosyphon Heap Pipe Evacuated Tube Collector