Thin films are deposited from lutetium bisphthalocyanine (LuPc(2)) and lutetium phthalo-naphthalocyanine (LuPNc) that are intrinsic, easily oxidizable, molecular semiconductors. Oxidation of the films by NO2 or Br-2 vapours is studied by visible absorption spectroscopy on "neutral" and oxidized films and by in-situ measurements of conductivity and sorbed dopant mass uptake, Exposures of the oxidized LuPc(2) and LuPNc films to NH3 lead to a conductivity increase or decrease depending on the oxidation degree of the films. The whole set of results is in agreement with a model that describes the existence of a conductivity maximum in LuPc(2) and LuPNc during oxidation as the consequence of both a trapping process of the charge carriers and a reduction of their mobility when the concentration of oxidized phthalocyanine molecules becomes too high.