This paper presents the development of a new process to desalinate seawater using solar energy. By the proposed process, the solar energy heats airflow up to a temperature between 50 and 80degreesC. The moderate solar heated air will be humidified by injecting seawater into the air stream. Later on, the water being free of salt will be extracted from the humid air by cooling it. Using air as a heat carrier and keeping the maximum operating temperature in the process lower than 80degreesC enables the use of cost effective polymers as construction material. The main feature of the present process is a successive loading of air with vapor up to a relative high humidity, such as 10 or 15 wt.%. As a result, the air volume flowing through the plant can be substantially reduced. This target will be realized by a new suggested stepwise heating/humidifying-technique. The thermodynamic background of the new process will be described. An optimizing procedure of the desalinating process by selecting of optimum process parameters will be explained. Low cost air heaters for collecting of solar energy are developed. Special designs for air humidification by evaporating of seawater has been constructed and tested. Condensing equipment has been designed to recover desalinated water out of the humidified air. This new equipment will be described and the test results of its performance will be delivered. The new collector types and the developed humidifying and dehumidifying equipment are a part of an indoor one-stage-plant consisting of a solar simulator and an air humidifying loop in Bochum, Germany. This plant is running now and serves as a pilot to provide optimum operating conditions and design guidelines for a demonstration plant. Based on these results, an engineering package for the demonstration plant will be completed.