Abstract: An osmometer for use in the measurement of gases, liquids and solids is disclosed. The osmometer includes a semipermeable hollow fiber, capable of passing liquids but not dissolved solids. The hollow fiber contains a media component with a predetermined osmolarity compatible with the sample to be measured. A gas volume is also contained within the hollow fiber and a sensor is present and located adjacent to the gas volume. The sensor is responsive to changes in the gas or media volume due to changes in the media component caused by the influx or efflux of water to the hollow fiber.