In a device for measuring redox gases, such as alcohol, in a person's breath, a disposable tubular mouthpiece forms an elongated flow passage and is displaceably securable in a measuring head. The mouthpiece has an opening at one end for blowing air into it and openings along its length for directing the air into the measuring head. A check valve is located in the flow passage so that air exhaled into the opening in the end of the mouthpiece can flow to the measuring head, but inhaling on the mouthpiece places the check valve in a closed position. The mouthpiece can be formed of a single molded piece or two tubular portions secured together. The end of the mouthpiece opposite the open end is closed and is spaced from the openings into the measuring head for forming a moisture chamber for collecting saliva.