The changes occurring on cellulosic fibers caused by ultraviolet irradiation at 360 nm for 24 h and 48 h were investigated. A never-dried industrial Eucalyptus urograndis elemental chlorine free–bleached kraft pulp sample (O/OD[EP]DP sequence) was used throughout the work. After irradiation, the cellulosic fibers were analyzed for polymerization degree changes, extractive and pentosan content, ultraviolet and visible spectrometric changes of water leachate from the fibers and thermal analysis (TGA/DTA). The ultraviolet treatment had no effect on the degree of cellulose polymerization. The water extracted from ultraviolet-treated fibers revealed cellulose oxidation, with increasing absorbance signals for carbonyl and carboxyl groups. Those oxidations influenced fiber thermal stability.