This paper presents a method to estimate the Arrhenius parameters as well as a reaction model of the pyrolysis of polystyrene (PS) from isothermal kinetic results. We used a state-of-the-art thermobalance (TB) that is able to continuously monitor weight decreases with respect to time under pure static conditions. A best fit of the theoretical reduced-time plot (RTP) to the experimental one led to the conclusion that the reaction model for PS pyrolysis within a reaction temperature range of 658~673 K would be accounted for by the "Avrami-Erofeev" model, n(1-α)[-1n(1-α)(n-1)/n, where n has an average value of 2.25 from four measurements estimated at 658, 663, 668, and 673 K. The n value increased slightly upon increasing the reaction temperature, possibly suggesting that PS pyrolysis may be represented by a complex reaction that may be accounted for by a series of decomposition reactions of PS intermediates. The Arrhenius parameters obtained from the model-fitting method using the Avrami-Erofeev model agreed well with the values from the model-free methods obtained isothermal measurements; hence, these results properly uphold the correctness of the kinetic triplet that we determined.