The effect of carbon dioxide (CO(2)) concentration on the ignition behaviour of hydrocarbon and CO(2) gas mixtures is examined in both jets and confined explosions. Results from explosion tests are presented using a 201 explosion sphere and an 8 m long section of 1.04 m diameter pipeline. Experiments to assess the flame stability and ignition probability in free-jets are reported for a range of different release velocities. An empirically-based flammability factor model for free-jets is also presented and results are compared to ignition probability measurements previously reported in the literature and those resulting from the present tests. The results help to understand how CO(2) changes the severity of fires and explosions resulting from hydrocarbon releases. They also demonstrate that it is possible to ignite gas mixtures when the mean concentration is outside the flammable range. This information may be useful for risk assessments of offshore platforms involved in carbon sequestration or enhanced oil recovery, or in assessing the hazards posed by poorly-inerted hydrocarbon processing plant. Crown Copyright (C) 2011 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. All rights reserved.