Poly-dopamine (PDA) recently came into focus as an innovative material for applications in various technical fields, especially in the energy field it is considered for batteries as well as for supercapacitors. It is a very strong glue that sticks on almost everything, even under water: In nature, PDA is found in the byssal thread cuticles of mussels. The wet-resistant adhesion ability of PDA can be used in the treatment of separator membranes in batteries, but furthermore PDA is also considered as an electrode material for batteries. We have investigated thin layers of poly-dopamine with different methods including ellipsometry, tactile measurements of the layer thickness and in situ absorption measurements during the polymerization process.