Abstract: The invention relates to the marking of textile products that have been dyed using a supercritical dyeing process. More particularly, the invention provides a process of marking a textile substrate with a tracer, said process comprising contacting the textile substrate with a supercritical or near-critical dyeing medium containing a dye and a tracer, said tracer being selected from a metal chelate and a chemiluminescent agent. The process of the present invention uses the supercritical or near-critical dyeing medium as a vehicle for depositing a tracer onto the textile substrate that is also dyed using this medium. The presence of the deposited tracer on the dyed textile can be detected using a simple and reliable authentication method. The invention also concerns a marked textile product obtained by the aforementioned process. Also provided are a dyeing composition that can be used in the aforementioned process as well as methods for authenticating textile products that have been marked using his process.