Experiments and direct numerical simulations are used jointly to study the asymmetric and symmetric time-periodic regimes occurring in a T-shaped micro mixer for larger Reynolds numbers than those of steady regimes. The first is characterized by a large mixing degree, whereas the flow in the second regime always exhibits a nearly double mirror symmetry in the mixing channel, which strongly hampers mixing. The characteristic nondimensional frequency, calculated using the hydraulic diameter of the mixing channel and the bulk velocity, augments with the Reynolds number in both periodic regimes, but a large discontinuity is observed at the transition between the two regimes. A detailed description of the main flow features is given to provide a physical explanation on the Strouhal number variation. The present analysis can be exploited in practice to design active control strategies, e.g., by exciting the flow at the frequencies typical of the asymmetric unsteady regime.