The aim of this study is to functionalize cellulose acetate (CA) fibers with an amine function by the adsorption of branched polyethyleneimine (bPEI) into the fiber surface. The adsorption was carried out by immersing CA tow in branched bPEI solution and washing off with water to remove any non-adsorbed bPEI. The mass increase and X-ray Photoelectron Spectroscopy (XPS) quantified the amount of bPEI adsorbed on the surface of the CA fibers. The effects of the coating duration, bPEI molecular mass, bPEI concentration, bPEI solvent and plasma treatment on the amount of the bPEI adsorbed and the amine density on the surface of the CA fibers have been investigated and reported here. The surface morphology of bPEI-treated CA fibers, compared to one of native CA fibers, was investigated by both Electron Probe Micro-Analyzer (EPMA) coupled with X-ray mapping, which is an important tool to analyze surface chemical composition, and Scanning Electron Microscopy (SEM), in order to illustrate the morphology of the CA fibers after bPEI adsorption as well as the plasma effect on adsorption process. The evaluation of bPEI coated-CA fibers for the adsorption aldehydes was carried out by exposing them to formaldehyde vapor generated from a formaldehyde solution. The adsorption mechanism was investigated by Cross Polarization - Magic Angle Spinning 13Carbon Nuclear Magnetic Resonance (CP-MAS 13C NMR) which proved that the aldehydes are chemically attached to the surface of CA fibers in the form of hemiaminals. Finally, the shelf life of plasma-treated CA fibers coated with bPEI was studied and reported.