Abstract: A carrier gas is passed through a liquid-repellent porous partition tubing having channels extending through the wall of the tubing and immersed in the liquid to be tested, causing a gaseous or volatile substance to permeate through the wall and diffuse into the carrier gas in the tubing. The carrier gas flowing out from the outlet of the tubing and containing a quantity of the substance in equilibrium with the liquid phase is led to a detector connected to the outlet, whereby the concentration of the gaseous or volatile substance in the liquid can be detected continuously or intermittently with high efficiency. The tubing, which is liquid-repellent, prevents ingress of the liquid into the channels but permits the gaseous or volatile substance to diffuse into the carrier gas through a gas layer in the channels at an exceedingly high velocity, thus affording measurements with a short response time.