AbstractA new method is described for investigating the osmotic situation of nonvolatile compounds in various solvents with a concentration limit as low as about 10−3 mole/1. The method is based on measuring as a function of time the loss of weight of an evaporator made of porous material wetted with the pure solvent, hanging in the vapour of the solution under investigation. The gas‐free solution is injected by its own vapour pressure through a crystal‐seal in an evacuated glass apparatus, having no ground joints or valves. Calibration data are given.