In the first part of this study, experimental (hot–wire and visualization) results on the effect of transverse acoustic oscillations on characteristics of round and plane air microjets at low Reynolds numbers and on their propane and hydrogen combustion are presented. In both cases, the transverse acoustic excitation results in the flame bifurcation. A phenomenon observed at the nozzle exit is the so-called «bottleneck flame». This is found both at acoustic excitation and without it at combustion of hydrogen. More details on this phenomenon are given in the second part of the study titled as: “Features of diffusion combustion of hydrogen in round and plane high-speed microjets (Part II)”.