Potassium formate and potassium bicarbonate are currently subject of research in a variety of new potential technical applications. One example of this is the use of the mixture of both salts in solution as a reversible hydrogen storage system and thus as a potential renewable energy carrier. In order to develop these new technologies, knowledge of the thermophysical properties of the individual salt solutions and mixtures is essential. Due to the limited data available in literature, which are only available for the single salt solutions, measurements of density and speed of sound as well as mathematical correlations based on the data were carried out in order to predict the behaviour of the salt solutions as a function of concentration and temperature. Good approximation equations were found for both properties, which agree with the existing data in the literature and result in deviations well below 1 % over wide concentration and temperature ranges. The large number of measurements, the clear trends in the results and the good agreement with existing data from the literature allow to assume that reliable predictions of concentration, and thus degree of hydrogen charging, can be realized utilizing density measurements. With the help of this fundamental research, it will not only be possible to investigate the newly discussed applications in more detail in the future, but also enable more precise design calculations, models and simulations. In addition, material properties that result from the measured variables or are related to them, such as thermal expansion or isentropic compressibility, can be analysed.