A two-stage dilution system and an associated procedure to simultaneously calibrate both open-path (long-path) and conventional point air monitors have been used successfully during a comparison test study of open-path monitoring systems in Houston during August, 1993. Two open-path, differential optical absorption spectrometers (DOAS) were calibrated using standard concentrations up to 50 ppm in a 0.9 meter stainless steel optical cell connected to each DOAS analyzer via fiber optic cables. The calibration apparatus consisted of various flow controllers, flow meters, and mixing chambers to provide accurate dynamic flow dilutions; a high-concentration ozone generator; a suitable reaction chamber for gas phase titration (GPT) of NO to generate NO2 standard concentrations; and an output manifold. High-concentration standards for SO2 and NO were obtained by the primary dilution of nominal 1000-ppm, NIST-traceable concentration standards in compressed gas cylinders. NO2 concentrations were generated by GPT from NO concentrations. The ambient-level concentration standards were provided by quantitative secondary dilution of the high concentration standards. Ozone standard concentrations were generated by the high-concentration ozone generator and assayed, after secondary dilution to ambient-level concentrations, by a commercially available UV photometric ozone analyzer used as a transfer standard.