The process by which a molecule in an intense laser ?eld ionizes more ef?ciently as its bond length increases towards a critical distance Rc is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for CO2 , by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We ?nd that for the 4 nd higher charge states, 100 fs is the time scale required to reach the critical geometry hRCO i ?? 2:1 ?? A and h??OCO i ?? 163?? (equilibrium CO2 geometry is hRCO i ?? 1:16 ?? A and h??OCO i ?? 172?? ). The CO2 3 olecule, however, appears always to begin dissociation from closer than 1.7 Aࠩndicating that dynamics on charge states lower than 3 s not suf?cient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO2 and identify the electronic states responsible for CREI.