The precipitation behavior of the nickel-base alloys Ni-Cr-22 Fe-18 Mo (Hastelloy-X) and Ni-Cr-22 Co-12 Mo (Inconel-617) has been investigated as a function of aging temperature. Hastelloy-X shows that M6C and TiN are primary precipitates and M12C, A3B2 (≈ Fe3Mo2), and M23C6 are secondary precipitates, while Inconel-617 also has M6C and TiN as primary precipitates and M23C6, M12C, and Ni3AlTi as secondary precipitates. The characterization has been carried out by metallographic and transmission electron microscopy investigations and by x-ray examinations of electrochemical isolated precipitates.