Nylon fiber was subjected to four trials of dyeing using different redox systems: pretreatment with a redox system, simultaneous dyeing and treatment, pretreatment with reductant, and/or pretreatment with oxidant. It has been claimed that dyeability improvement in dyeing of nylon fibers using a redox system is attributed to covalent bonding; however, in this work, it is proved that the increase in dyeing rate and adsorption of acid dyes on nylon fibers in the presence of redox systems could be attributed to the decrease in pH that would lead to an ionic bond, but not to the covalent fixation of dyes on nylon fibers.