The objective of this work is an experimental study of the influence of initial and boundary conditions at the nozzle exit upon diffusion combustion of a hydrogen microjet. It is found that the initial mean velocity profile and the presence (or absence) of a material with large heat capacity surrounding the nozzle exit may have a pronounced effect on the flame structure and combustion of the round hydrogen microjet. The rates of fuel consumption (i.e., the efflux velocities of hydrogen) providing diffusion combustion of the round hydrogen microjet, the flame detachment, and the origination of the << bottleneck flame in the cases of a top-hat and a parabolic mean velocity profiles at the nozzle exit are determined. The variations of the extent of << bottleneck flame>> with the hydrogen flow rate are made clear. Also we examine the diminution of the extent of bottleneck flame with the growth of hydrogen flow rate in three cases of initial conditions at the nozzle exit. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.