Volatilizations of molybdenum and ruthenium during the fixation of simulated high-level radioactive waste in modified SYNROC-B crystalline ceramic waste forms have been studied using a radiotracer technique. The simulated waste loading was 20 wt%. The volatilization figures of merit (VFMs) for 99Mo and 103Ru have shown a behavior that depends on the type of sintering atmosphere, i.e., oxidizing versus reducing. The experimentally obtained VFMMo and VFMRu in an oxidizing atmosphere are 7.8 and 3.7% of the initial radioactivity of each nuclide per gram of sintered SYNROC-B product after sintering at 1510 K in air, while under reducing conditions (50% H2-50% Ar), VFMMo and VFMRu have been reduced to 2.8 and 1.8% g−1, respectively. Solidification of high-level radioactive waste in the proposed waste form or in glass matrices under reducing atmosphere is recommended to minimize the amounts of volatilization and, subsequently, to reduce the safety requirements of the off-gas treatment system of the vitrification and/or solidification plant.