In the present work, a preliminary study of the pyrolysis process of high density polyethylene (HDPE) in a fluidized bed is investigated in order to determine the influence between the fluidynamic properties of the bed reactor and the amount and composition of the gases produced. As is known, fluidized bed technology is a very interesting option to apply in the pyrolysis field due to i) the lack of moving parts in the hot region that facilitates the maintenance of equipment, ii) the high surface area to volume ratio available in the bed, and iii) the high heat transfer coefficient reached which governs the reaction products. But, heat and mass transfer coefficients are strongly affected by the fluidynamic properties of the bed. During the pyrolysis of HDPE, a fluidynamic characterization of the bed particles that consist of char-coated sand of HDPE has been carried out. Parameters such as the minimum fluidizing velocity (u(mf)), terminal velocity (U-r), bed height (h(f)), bed voidage (epsilon(f)), fraction of the bed occupied by bubbles (delta), bubble diameter (d(b)), bubble velocity (U-b), the mass transfer coefficients between the bubble and the cloud (K-bc) and between the cloud and the emulsion (K-ce) were determined. Subsequently, the influence of major operating variables and the fluidynamic parameters on the composition and the gas yield of the pyrolysis of HDPE were studied. (C) 2010 Elsevier B.V. All rights reserved.