Unsteady flow regimes in a micro T-mixer are investigated. Direct numerical simulations (DNS) show that, in agreement with experimental and numerical results in the literature, when the Reynolds number is increased above a critical value larger than those typical of the steady engulfment regime, the flow remains asymmetric in the mean but becomes periodic in time. The dynamics of this regime is characterized by analyzing the evolution of the three-dimensional vortical structures forming at the confluence of the two inlet flow streams. As the Reynolds number is further increased, the flowremains time-periodic but it continuously switches between a symmetric configuration, similar to that of the vortical regime, and an asymmetric one, close to the engulfment configuration. Three-dimensional linear stability analysis is successively used to characterize the instability leading to the unsteady asymmetric regime, which is also interesting for applications due to its high mixing efficiency. The critical Reynolds number and the instability frequency are in very good agreement with those found in DNS. The sensitivity of this instability to a generic perturbation of the base flow is also investigated. It is shown that the largest sensitivity is to base-flow modifications introduced close to the three-dimensional vortical structures forming at the confluence between the inlet channels. Finally, the sensitivity analysis is specialized to investigate the effect of a generic perturbation of the inlet velocity profile. (C) 2014 AIP Publishing LLC.