Rocky Flats Environmental Technology Site and other DOE sites have large volumes of low-level mixed waste (LLMW) in powdered forms, which must be processed into stable waste forms suitable for long-term storage. Polyethylene microencapsulation, in which the waste is thoroughly mixed with molten polyethylene and extruded into containers, is the first LLMW stabilization process that can be monitored in real-time to avoid processing errors and provide regulators with proof-of-proper-processing documentation. Monitoring compatibility and the smaller, lighter, more economical, inert polyethylene waste form results in significant advantages over cementation and grout based matrices, which are prone to chemical interaction with the waste. The monitor is a noncontact device that acquires the midinfrared spectrum of the process stream via emission spectroscopy. The stream composition is then derived from the spectrum using partial-least-squares techniques. Monitor operation is insensitive to the thickness of the process stream and to its surface morphology. The monitor has been demonstrated on polyethylene microencapsulation lines processing nitrate-salt LLMW and its surrogate, molten salt oxidation LLMW and its surrogate, and flyash. Typically, the monitor achieves a standard error of prediction of less than 1% by weight with an analysis time of less than 20 seconds.