<p>The experiments of hydrogen–air deflagration were conducted in a closed combustion chamber to investigate the effects of initial temperature and inert gas addition on the dynamic characteristics. The dynamic behavior of spherically propagating flames was observed by using high-speed Schlieren photography, and the flame radius and propagation velocity were measured by analyzing Schlieren photographs. At sufficiently small flame radii, we obtained the propagation velocity of unstretched flame. At large flame radii, cellular flame fronts were generated by intrinsic instability. Owing to the cellular shape of fronts, the propagation velocity increased, and then the flame acceleration was confirmed. We obtained the increment coefficient of propagation velocity. As the initial temperature became higher, the dynamic behavior of flame fronts weakened. Thus, the increment coefficient normalized by the propagation velocity of unstretched flame decreased. This was because of the weakness of intrinsic instability. Moreover, the normalized increment coefficient increased as the inert gas concentration became higher, especially in the case of carbon dioxide as inert gas. The dynamic characteristics of hydrogen–air deflagration were greatly affected by the initial temperature and inert gas addition.</p> Graphical abstract <p></p>