Research into the performance of proton exchange membrane fuel cell (PEMFC) and its degradation ("poisoning") by the presence of carbon monoxide, a common byproduct of most common hydrogen production methods, requires storage of a large quantities of hydrogen/carbon monoxide mixture in high pressure tanks. The possibility of unintended release of the gas calls for a safety study of H-2/CO mixture leaks, as well as potential higher rate releases. This presentation covers the safety aspects of the numerical study of a H-2/CO mixture release at a wide range of release parameters, including release velocity, orientation, initial diameter, and initial gases fraction ratio. The study provides a simulation of a) the extent of flammable concentrations of H-2, corrected for the presence of CO; b) the extent of CO concentration exceeding OSHA recommended health safety limits. The presence of CO in the mixture required reassessment of hydrogen flammability limits, although the correction proved to be relatively small for the CO fractions used in the present. The maximum extents and evolution of H-2 and CO envelops had been modeled and compared. An important conclusion is the possibility of using hydrogen detectors to predict CO concentration levels with accuracy sufficient for practical purposes. (c) 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association