Fourier transform infrared spectroscopy, scanning electron microscopy, and the weight-loss method were utilized to propose a novel kinetic model for PMMA decomposition by microwave digestion with HNO3 as a digestion reagent. By Newton's method, the experimental results of the remaining weight of PMMA is closely fitted by the model combined with zero-order and first-order kinetics, in which the former dominates the reaction at lower temperatures (423-443 K) and the latter at higher temperatures (>= 453 K). Accordingly, a mechanism for PMMA decomposition is proposed in this study. Kinetic parameters of PMMA decomposition under 423-453 K including rate constants and the mass fractions (alpha) via main-chain scission were determined by this model. Activation energies of PMMA decomposition estimated by Arrhenius equation are 2.45-2.81 and 23.5-27.0 kcal mol(-1). respectively, for the zero- and first-order reactions. The pre-exponential factors of the zero- and first-order reactions are 1.96 x 10(-2) to 2.92 x 10(-2) min(-1) and 1.31 x 10(11) to 7.08 x 10(12) min(-1), respectively. Effect of HNO3 volume on PMMA decomposition was investigated at 423-473 K. The digestion efficiency apparently increases as HNO3 volume is more than 3 mL at these temperatures. At 473 K, the digestion efficiency has increased to 100% as HNO3 volume is >= 3 mL. The estimated alpha values of the decomposition with 2-7 mL of HNO3 at 423-473 K are increasing with HNO3 volume at 423 and 443 K, yet varying insignificantly at 453 and 473 K. (C) 2007 Elsevier B.V. All rights reserved.