The present invention relates to apparatus for determining a relevant proportion (a) of a chosen gas, an NO-gas or NO.sub.2 -gas, present in a gas mixture (A), such as in a gas mixture deriving from the expiration air or inspiration air of a living being (1), wherein an adapted sample proportion of the breathing air is passed from an outlet (11) in the immediate vicinity of said being, via a hose (10') or the like, to measuring equipment (13') that includes a measuring chamber (14') that is placed under a subpressure and to which ozone gas is also supplied, wherewith the nitrogen oxide present reacts with the ozone-gas to form NO.sub.2 and light, and wherein the apparatus includes a light measuring unit (13') which is adapted to evaluate the light thus generated and to calculate (18) the amount of predominant gas in the breathing air on the basis of this evaluation. The volume (14b') of the measuring chamber of such apparatus is adapted to include the volume enclosed in the hose (10') used. The hose-related valve (11a') or constriction that delimits the subpressure volume of the measuring chamber is provided in the close proximity of the being (1). A means (30) for reducing the water vapour in the breathing air sample (A), e.g. by cooling and/or freezing, is adapted upstream of the measuring chamber (14') to reduce or eliminate precipitation of water in the measuring chamber.