The gas-liquid hydrodynamics in a Sulzer SMX TM static mixer was investigated in the present work through two different optical techniques: Backlight Shadowgraph Technique (BST) and Particle Image Velocimetry (PIV). 3D printed static mixers were manufactured using transparent plastic in order to provide optical access. The normal-heptane was used as the continuous liquid phase. Three different lengths of mixers and different gaseous nitrogen flow rates were investigated. The flow pattern in an empty tube without the mixing device was used as a reference. Bubble diameter distributions at the inlet and outlet of the SMX mixer were evaluated. The velocity fields inside the mixers were quantified. The gas holdup was also examined. These original results allow to appreciate the SMX static mixer's performance and thus to open new industrial applications involving gas-liquid flows such as striping and purification of liquids by a gas