Distributions of hydrogen gas concentration in a suddenly started, single shot hydrogen gas jet issuing from a 1 mm diameter injector into still air were measured using laser interferometry method. This unsteady, turbulent free jet flow has also been calculated using the two-equation, high Reynolds number version of k-ε turbulence model and hybrid scheme for treating combined diffusion and convection in the SIMPLE algorithm. The injection pressure was 0.5 MPa for which predicted and measured temporal jet tip penetration distributions indicate that the jet discharged into still air at Mach 0.25. The level of agreement between present prediction and measurement is good in some regions and poor in others.