Radon can dissolve and build up in water from underground sources. If water comes from a well, it may contain Radon. Radon in the water dissolves and escapes into the air during household water use, especially when it is heated. An easy-to-handle experimental setup for the on-site determination of the Radon concentration in water samples has been developed and tested. In contrast to laboratory techniques for Radon-in-water determination such as liquid scintillation counting or gamma-ray spectroscopy, the described equipment and method allow for a fast and uncomplicated determination of the Radon concentration in water samples immediately on site with satisfactory accuracy. The method is based on the water/air partitioning of Radon, which is known quantitatively over a wide range of temperatures. Another set-up has been designed for on-line Radon monitoring in water using the newest Radon monitor AlphaGUARD DF2000.