The viability of thermal treatment using vitrification to immobilize two types of Sellafield intermediate level wastes (ILW) has been successfully demonstrated in proof-of-concept trials. The testing employed a type of Joule Heated Ceramic Melter (JHCM) technology that incorporates active mixing of the melt pool. Melt pool mixing improves heat and mass transport, increases waste processing rates, thereby reducing systems size and/or operating duration, and therefore overall treatment costs. Simulants for two Sellafield ILW streams—a Magnox sludge and a sand/clinoptilolite ion exchange waste slurry—were developed for testing in this work. Glass formulations were developed and tested for each of the two ILW streams. Acceptable glass formulations that met all processability and product quality constraints were selected for testing on a small-scale continuously-fed vitrification system. Two tests of nominally 50-hour duration were completed. Data were collected to characterize operating conditions, processing rates, and glass and off-gas compositions for mass balance. Key performance objectives included minimum nonactive additives; maximum volume reduction for the wastes; maximum retention of radionuclide and chemotoxic elements; minimal secondary wastes that require other processing; maximum passivation and stabilization of wastes; and meeting product compliance requirements. Test results demonstrate that vitrification using JHCM technology is well suited to immobilize the two Sellafield waste streams tested. The required system size and operating duration are well within the realm of prior experience of this technology. The technology offers the potential to significantly reduce lifecycle cost because the high volume reductions minimize the volume of treated wastes to be disposed, while the glass waste form offers superior waste form performance compared to almost all alternatives.