Intrinsic carbon doping of GaAs (001) during metal-organic vapour-phase epitaxial (MOVPE) growth using low V/III ratios was investigated by in situ reflectance anisotropy spectroscopy (RAS). The surface reconstruction during growth changes from arsenic-rich (1 x 2)-CH3 at high V/III ratios to gallium-rich (1 x 4)-CH3 at very low V/III ratios. This change in reconstruction corresponds to three different incorporation regimes. For growth at low V/III ratios ((1 x 4)-CH2) the hole concentration saturates at approximate to 4 x 10(19) cm(-3) and the surfaces are smooth. At intermediate V/III ratios (transition between (1 x 4)-CH2 and (1 x 2)-CH3) the doping level decreases following a power law dependency and the surface becomes rather rough. At very high V/III ratios ((1 x 2)-CH3) the surfaces become smooth again and the doping saturates at a temperature-dependent background doping level. For the growth at very low V/III ratios a new methyl desorption/arsenic adsorption limited growth mechanism is proposed, characterized by a very high carbon incorporation and smooth surfaces.