The in situ chlorination of graphene on Ir(111) has been achieved by depositing FeCl3 followed by its thermal decomposition on the surface into FeCl2 and Cl. This process is accompanied by an intercalation of Cl under graphene and formation of an epitaxial FeCl2 film on top, which can be removed upon further annealing. A pronounced hole doping of graphene has been observed as a consequence of the annealing-assisted intercalation of Cl. This effect has been studied by a combination of core-level and angle-resolved photoelectron spectroscopies (CL PES and ARPES, respectively), near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and low-energy electron diffraction (LEED). The ease of preparation, the remarkable reproducibility of the doping level and the reversibility of the doping upon annealing are the key factors making chlorination with FeCl3 a promising route for tuning the electronic properties in graphene.