Very little is known about the health hazards related to the presence of tattoo inks in the human body. The lack of research data on the tattoo inks toxicity is particularly worrisome since it is widely accepted that numerous key ingredients of the tattoo inks are potentially toxic and might be even carcinogenic. In this context, several pertinent questions need to be raised, including: (i) what are the ingredients of the commercial tattoo inks, (ii) what is their contaminant content, and (iii) what are the mechanisms of the adverse processes the tattoo ingredients might cause in the human body? This work was oriented towards gaining mechanistic insights into the possible toxicity of the whole tattoo inks in vitro, as well as to single out the potentially most toxic ingredients. Several analytical techniques were implemented, including: UV-VIS spectroscopy, electron spin resonance (ESR), scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), electro-spray ionization mass spectrometry (ESI-MS), and X-ray powder diffraction. This study confirmed the visible-light-induced formation of reactive oxygen species (ROS) in all the studied tattoo inks and also demonstrated that tattoo inks containing traces of iron might also cause oxidative processes via the Fenton reaction mechanism.