Metal fluorides generally demonstrate a wide band gap and a low refractive index, and they are commonly employed in optics and optoelectronics. Recently, an SF6 plasma was introduced as a novel co-reactant for the atomic layer deposition (ALD) of metal fluorides. In this work, the reaction mechanisms underlying the ALD of fluorides using a fluorine-containing plasma are investigated, considering aluminum fluoride (AlF3) ALD from Al(CH3)3 and an SF6 plasma as a model system. Surface infrared spectroscopy studies indicated that Al(CH3)3 reacts with the surface in a ligand-exchange reaction by accepting F from the AlF3 film and forming CH3 surface groups. It was found that at low deposition temperatures Al(CH3)3 also reacts with HF surface species. These HF species are formed during the SF6 plasma exposure and were detected both at the surface and in the gas phase using infrared spectroscopy and quadrupole mass spectrometry (QMS), respectively. Furthermore, QMS and optical emission spectroscopy (OES) measurements showed that CH4 and CHyF4-y (y ≤ 3) species are the main reaction products during the SF6 plasma exposure. The CH4 release is explained by the reaction of CH3 ligands with HF, while CHyF4-y species originate from the interaction of the SF6 plasma with CH3 ligands. At high temperatures, a transition from AlF3 deposition to Al2O3 etching was observed using infrared spectroscopy. The obtained insights indicate a reaction pathway where F radicals from the SF6 plasma eliminate the CH3 ligands remaining after precursor dosing and where F radicals are simultaneously responsible for the fluorination reaction. The understanding of the reaction mechanisms during AlF3 growth can help in developing ALD processes for other metal fluorides using a fluorine-containing plasma as the co-reactant as well as atomic layer etching (ALE) processes involving surface fluorination.