An experimental and theoretical study has been performed on the formation of NO in the combustion of volatiles from municipal solid wastes. Single components and their mixtures were burned in a small-scale, fixed-bed reactor. Numerical simulations using the opposed flow diffusion flame program OPPDIF were performed to obtain a further understanding of the experimental results. Conversion factors for fuel-N to NO were determined for single components of newspaper, cardboard, glossy paper, low-density polyethylene (LDPE), and poly(vinylchloride) (PVC) and their mixtures, using gases with oxygen concentrations of 12, 21, and 40 vol %. For single components experiments at 100 vol % oxygen were also performed. The conversion factors for paper and cardboard varied from 0.26 to 0.99. The conversion factor for LDPE and PVC varied from 0.71 to 10.09 and 0.04 to 0.37, respectively. Conversion factors higher than 1.0 in the case of LDPE clearly show that NO is formed by thermal and/or prompt mechanisms. For mixtures, calculated conversion factors (based on a weighted sum of the conversion factors for single components) were compared with the experimentally determined conversion factors. Mixtures of paper and cardboard only gave different conversion factors with 40 vol % of oxygen. For mixtures of paper/cardboard and plastics, however, significant differences in the conversion factors were observed at all oxygen concentrations, when comparing experiments on a mixture of paper and plastics with the weighted sum of the single components. The explanation is found in the different combustion properties for paper/cardboard and plastic, which in this case make the formation of thermal NO from LDPE more favorable for the single component than in mixtures with other components. The simulations with OPPDIF confirmed the trends observed experimentally, and allowed an assessment of the contribution of the different mechanisms of NO formation. (C) 2001 by The Combustion Institute.