The Mn dopant distribution in ultrathin (20 nm) highly doped (nominal x = 0.20) Ga1-x Mn-x As epitaxial films with critical temperatures close to 175 K and magnetization of 100 emu/cm(3) is analyzed by Rutherford backscattering spectrometry (RBS) in a random and channeling configuration. We could quantify the total concentration and the respective fraction of substitutional, interstitial, and random site Mn ions in as-grown and annealed samples. The measured total Mn concentration is x = 0.23. In the as-grown state 30% of the Mn dopant is located on interstitial sites. Thermal annealings at 180 degrees C for several hours monotonically reduce the interstitial Mn fraction to 11%. Simultaneously the fraction of randomly located Mn is increased by the same amount. The substitutional Mn concentration is stable under these annealing conditions. The effective Mn concentration could be increased to x = 0.13. However, the critical temperature does not increase proportionally with the magnetization. A comparison of the magnetization values demonstrates that the interstitial Mn ions are already incorporated during the growth in the form of Mn-Ga-Mn-i clusters.