[en] Carrier density profile determinations are reported which confirm that a mass action relation holds between the ionized and the inactive mobile As in silicon. Experiments were performed under equilibrium conditions at temperatures in the range 780-885 deg. C. The carrier density n vs. the dopant concentration N_T exhibits a very pronounced saturation behaviour which is compared with the one expected in well known cluster models. These results complement the recent finding that the inactive As can exist in equilibrium with the monoclinic SiAs. From this information and previous SAXS, TEM, and EXAFS results one can conclude that a pre-precipitation phenomenon, with its inherent cluster distribution, is responsible for the inactive As. (author). 21 refs, 2 figs