The behavior of pulp and paper sludge fibers during oxidation in air was investigated experimentally using a high-temperature thermogravimetric system (TGA). A sludge sample (1.0 g) was heated in the TGA furnace at a rate of 40 degreesC/min up to a specified maximum temperature at which it was kept for 10 min. The sample was then cooled to room temperature at 5 degreesC/min. Experiments were performed for different maximum temperatures ranging from 750 degreesC to 1500 degreesC. The influence of the thermal input on the sludge mass decay and the effect of temperature on the physical changes in the sludge structure were determined. It was found that the combustion of sludge fibers took place in three stages. The transition from one stage to the other was identified by a change in the slope of the mass loss curve. Stage one (temperature < 400C) was dominated by devolatilization with a constant mass loss rate. Almost 68% of the total sample mass loss occurred in this stage. Stage two (temperature > 400 degreesC) was dominated by char oxidation with a nonlinear mass loss rate. Stage three entailed the cooling period where no significant additional mass loss was observed. Sludge and ash samples obtained at different temperatures during the oxidation process were analyzed using quantitative scanning electron microscopy. Based on the physical properties of the sludge/ash samples, a new parameter delta (c) called "Dynamic Heat Factor" was determined. Since delta (c) takes into account the morphological changes of the sludge fibers as they decompose and provides an indication of the susceptibility of this material for further decomposition, it should be useful in the calculations of sludge waste combustion. It was found that delta (c) drops sharply and linearly at temperatures below 600 degreesC and decreases exponentially after that. Thus, the susceptibility of the sludge fibers to decompose decreases continually during the oxidation process while their resistance to thermal input increases. Calculations have shown that the optimal thermal input was around 32,000 degreesC min. This thermal input provided the maximum toss of the sludge mass at optimum time and temperature.