The transition phenomena in free jet diffusion flames of pure hydrogen and hydrogen-nitrogen mixtures were studied in unconfined still air using the schlieren technique. The break-point lengths of both the inner fuel jet and outer flame zone were measured as the jet exit Reynolds number was varied by using different diameter burner tubes of 1.42, 1.97, and 2.39 mm and by varying the nitrogen concentration. Two distinct types of the diffusion flame jets were found. The break-point length-jet exit Reynolds number dependence of the flames with large kinematic viscosity (vj > 0.6 cm2/s) exhibits four distinct regimes, controlled by (1) minimum Reynolds number for instability, (2) free jet instability, (3) flow transition in the feed tube, and (4) turbulence in the burner wake, associated with three possible "triggers.'" The dependence of the flames with small kinematic viscosity (vj < 0.6 cm2/s) lacks regimes associated with the laminar jet instabilities and is composed of only two regimes. It is shown that the most important parameter is the Reynolds number based upon the local gas dynamic parameters at the break-point and that the critical condition at which a transition to turbulence in a laminar jet occurs can be related to a threshold value of this effective Reynolds number.