The degree of mixing of a T-shaped micro-mixer depends strongly on the inlet flow conditions. Specifically, through a series of numerical simulations, we compared the case where the flow at the micro-mixer confluence is fully developed with that when it is not, and found that in the former case engulfment occurs at smaller Reynolds number, with a different flow pattern and with much larger mixing efficiencies than in the latter case. In particular for fully developed flow conditions the engulfment shows S-shaped morphology, whereas for nonfully developed inlets a symmetry breaking occurs, leading to a C-shaped engulfment pattern. This is characterized by much lower mixing efficiencies.