An experimental study has been performed to determine the factors affecting the stability of hydrogen sulfide calibration mixtures at concentrations below 1 ppm. Quantitative data was obtained that illustrates that material selection and surface treatment are major factors affecting the shelf life of low concentration reactive gas mixtures. With the proper material selection and surface treatment, hydrogen sulfide and carbonyl sulfide standard mixtures on the order of 50-100 ppb can be maintained for at least 18 months.