One of the most important instrumental correction factors in the measurement of stable isotope ratios from dynamic gas source mass spectrometers is the so-called mixing correction for the changeover valve of the inlet system. Methods for the determination of this parameter are discussed and it is demonstrated that with a particular valve a commonly used technique gives values of the correction factor which are not necessarily applicable to normal operation. A more rigorous procedure is suggested, and details are given of the mixing investigated with argon, carbon dioxide and nitrogen.