This paper shows the comparison of the models for the in-line determination of the concentration of aqueous lithium bromide in the components of photo-thermal systems. For this study several operational conditions at different concentrations, pressures and temperatures have been considered. The behaviors of the physical properties have been analyzed as a function of the temperature. The results show the advantage of the use of the refractive index compared to the data of the thermal conductivity. The in-line determination of LiBr concentration allows calculations to improve optimal recovery of solar heat in heat transformers or air conditioning systems. (c) 2006 Elsevier B.V. All rights reserved.