•The flow in a CIJs mixer is studied with the PLIF flow visualization technique.•Three flow regimes are observed in the range 50<Re<600.•The jets’ flow imbalance always influences negatively the mixing quality.•The mechanism of eddy engulfment is a key aspect in the reduction of mixing scales. The flow in a Confined Impinging Jets (CIJs) mixer was studied with Planar Laser Induced Fluorescence. Two key aspects influencing the flow regimes and mixing quality were studied: the impact of the jets’ Reynolds number (Re) in the range 50<Re<600; and the effect of the jets’ flow imbalance, maintaining the flow rate of one of the injectors fixed and varying the other. Mixing mechanisms and scales are studied from the acquired flow images, and the mixing degree is quantified from the calculation of the Intensity of Segregation (Danckwerts, 1952). In balanced flow conditions, i.e., equal volumetric feeding rates, when the best mixing performance is observed, three flow regimes are observed: for Re<103 the flow is steady with complete segregation of the two feeding jet streams; for Re=104 the flow tends to an oscillatory periodic laminar flow regime; for Re>104 the flow evolves to a self-sustained chaotic laminar regime with strong mixing dynamics. With the increase of Re it is observed the formation of smaller mixing scales in the flow and an increase on the mixing quality. The visualized mixing scales are compared with theoretical models existing in the literature for the estimation of the striation thickness, leading to the conclusion that, in the studied flow regimes, the statistical theory of turbulent diffusion does not provide a realistic physical description of the flow. Furthermore, the jets’ mass flow imbalance is shown to always influence negatively the mixing quality. This is observed even when the flow imbalance results in the increase of the flow rate of one of the jets, increasing the amount of energy supplied to the system for dissipation. Results of this work show that the mechanism of eddy engulfment promoted by the two chaotically oscillating impinging jets, in the laminar regime, is a key aspect in mixing by CIJs.