The onset phase between combustor initiation and the appearance of a stable, periodic operating state is studied for four combustor conditions. The onset behavior and duration is assessed for varying air flow rate and equivalence ratio for cases with and without a converging nozzle at the combustor exit. The median duration and scatter of the onset phase is a function of flow rate and the combustor exit geometry, with higher air flow rate or the presence of a converging nozzle yielding rapid onset of a stable operating mode. The duration of the onset phase is very stochastic and varies by a factor of two for cases without a converging nozzle. However, the combustor behavior during this transitory period is very repeatable between subsequent tests.