Solubility of nitrogen in propylene glycol/water mixtures (25 %, 41.84 % and 100 % glycol by weight) have been determined in a temperature range from 10 °C to 110 °C using a static isochoric measuring method and are compared to the solubility of a typical solar liquid. To evaluate the amount of gas in a solar system it is necessary to measure the part of gases dissolved in the solar liquid, but also to consider the fraction of free gases (gas bubbles or gas cushions) in the system. Measuring this part was either not possible or only possible with a big technical effort until now. Therefore a new method was developed, which easily and fast identifies the volume of undissolved gases ("Gas Bubble Test"). The method was validated in extensive investigations in real solar systems.