Determination of isotopic composition of strontium and Rubidium in some geological samples was performed by using thermal ionization mass spectrometry. The optimum evaporation and ionization currents were determined for each of strontium and Rubidium, employing double rhenium filament by using faraday cup as detector. The results were compared with reference mater ials (NBS Standards) of Sr and Rb. The
radioactive decay of 87Rb to 87Sr was also determined.