Several studies on textiles have shown that a significant number of microparticulate fibers are released daily during washing and released into sewers and sewage treatment plants (WWTPs). These fibers can end up in the environment and have a potentially negative impact on ecosystems. The aim of the present study is to determine the biodegradability (mineralization) of microplastic fibers compared to natural (cellulose), regenerated (viscose), polyethylene terephthalate (PET) / cotton blends and oxo-degradable PET fibers under laboratory conditions with a 60-day period ready biodegradability test and an inoculum from a sewage treatment plant. In addition, the influence of various textile finishes on the biodegradability of fibers on viscose fibers that had been treated with dyes, a plasticizer and an antimicrobial agent was investigated. A high level of mineralization was observed in both cotton and viscose (up to 70%), while PET fibers were not degraded. Viscose fibers impregnated with certain finishing agents showed a much lower degree of degradation than untreated fibers, suggesting that the textile finishing could also extend the environmental half-life of fiber waste. The resistance of treated fibers to biodegradation can depend on the durability and type of finish. This is an important fact when it comes to going for more sustainable textile products and developing them chemically.