In this study, 15 mg samples of high density polyethylene (HDPE) were pyrolysed in an isothermal thermogravimetric set-up in a micro-thermobalance reactor with pre-set temperatures. The objective was to study the thermal degradation behaviour of the polymer and determine the kinetic rate constants and overall activation energy (E-o). Yielded pyrolysis products under different temperatures were analyzed and lumped into the following categories: rich gases (C-1-C-4), liquids (non-aromatic C-5-C-10), aromatics (single ring structures), waxes ( C-11) and char. It was observed that the gas yield was increasing with the operating temperature. Data obtained enabled us to propose a novel kinetic modelling approach based on the thermal cracking of the polymer by primary and secondary depolymerisation reactions to form the pyrolysis products. The E-o was estimated to be 147.25 kJ mol(-1) for the total conversion of HDPE in isothermal pyrolysis. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.