As an emerging technology, nanofluids can be used to split the solar spectrum for PV/T applications At present, most of the nanofluids developed for solar applications have been at low temperature, but it may be possible to develop fluids for high temperature systems. This study takes an in-depth look at the allowable temperature of the nanofluid and at how this directly affects the performance of hybrid system. In order to analyze this issue, the present work develops and experimentally validates a model to predict the performance of high temperature nanofluid-based spectral splitting PV/T systems for a wide range of optical properties, flow rates, and solar concentration ratios. The efficiency of system, the allowable temperature of fluid, and heat to electricity ratio are the main metrics of comparisons for this parametric analysis. It was found that the optimum 'cut-on' wavelength of nanofluid transparency should be set at 620-680 nm for silicon solar cells, for concentration ratios of 10-50, when the allowable temperature of nanofluid is 400 degrees C. This value will then move towards longer wavelengths as the allowable temperature of the nanofluid increases. The 'cut-off wavelength for nanofluid transparency should be the bang-gap of the cell, or 1100 nm in the case of silicon cells. When an optically ideal nanofluid is adopted, it was found that the power generated in the proposed hybrid system will come mainly from the thermodynamic conversion process. Therefore, we can conclude from a technical perspective that nanofluid-based PV/T systems should focus on developing fluids with as high as possible allowable temperature ratings.