Three pairs of the proton form of beta zeolites with similar Si/Al ratios but notably different crystallite sizes (10-1500 nm) were prepared in basic or fluoride media and tested as catalysts for the liquid-phase degradation of high-density polyethylene at 380 degrees C and atmospheric pressure. Among these, a nanocrystalline beta zeolite with the lowest Si/Al ratio (10.7) and the smallest crystallite size (similar to 10 nm) was found to show a high degradation activity and a remarkable liquid products yield (similar to 80%), together with high selectivities for C-7-C-12 hydrocarbons, which is rationalized on the basis of its very large external surface area and its high acidity compared to the other beta zeolites studied here. (C) 2008 Elsevier B.V. All rights reserved.