Experimental and theoretical results on the properties of CO2laser‐induced carbon and polyethylene (CH2) plasmas at laser intensities of ∼1015W/cm2are presented. The Thomson parabola technique is used to measure the ion velocity distribution in the underdense expanding plasma which is collisionless and isothermal. A model which treats the problem of the collisionless expansion of an isothermal electrostatic multi‐cold‐ion quasineutral plasma will be used to interpret the experimental results. Experiments and theory show that the effect of hydrogen in the CH2target induces a cutoff in the carbon ion velocity distribution. Theory suggests that acceleration field attenuation effects modify the behavior of these plasmas. Data suggest that the space–time evolution of the ion velocity distribution is completed before an ionization‐recombination equilibrium is reached. Experimental results from the underdense region are used to estimate plasma parameters near the critical surface, which show that the presence of hydrogen in the target apparently greatly reduces the thermal temperature near the critical surface, probably due to enhanced lateral energy transport.