At temperatures ranging from 300 to 450 degrees C and under atmospheric pressure, Ru-3/KY and Ru-3/NaY catalysts prepared from sorbing Ru(CO)(5) generated by Ru-3(CO)(12) on KY and NaY zeolites were found to be much more active for ammonia synthesis than Ru/KY and Ru/NaY prepared from cation exchange with Ru(NH3)(6)Cl-3. Dynamic secondary ion mass spectrometric and X-ray photoelectron spectroscopic studies disclosed that the reduced Ru metal particles were mainly entrapped within the zeolite framework and that their nature is related to the precusor forms.