NaCl solutions containing carrier-free125mTe(IV) and125mTe(VI) have been prepared. The effect of the presence of various substances on the colloidal behavior of125mTe in solution has been studied by ultracentrifugation. It has been found that phosphate, sulfide and oxalate ions which complex with various elements decrease the colloidal formation of125mTe. In order to estimate the size of colloidal particles, the NaCl solutions containing carrier-free125mTe have been microfiltered through membrane filters with various pore size.