Mechanical Biological Treatment inert reject (MBTr) is mostly composed by recyclable glass. Yet, it is usually sent to landfills because, due to the small particle size and high level of particle surface organic contamination, the processes commonly used in glass recycling, like optical sorting, are inefficient in the separation of glass from contaminants. This paper presents the results of a study undertook to characterize MBTr, aiming at the subsequent separation of recyclable glass from contaminants. A representative sample of the reject from a MBT plant was collected. The particle size analysis was carried out and the composition of the different size fractions was determined. It was found that the MBTr sample was composed by almost 80% of recyclable glass. The product is quite fine. About 80% in weight of the product has a particle size lower than 10 mm. However, the size fraction below 2 mm is insignificant since it represents less than 3% in weight and contains less than 1% of the recyclable glass in MBTr. Using methods of image analysis, the shape of each individual particle, separated by material class and by size fraction, was evaluated. It was observed that the average shape of the particles of "stone", the main contaminant, is significantly different from the shape of other materials, including recyclable glass. Gravity concentration methods may take advantage of the shape of the particles and are thus potentially adequate to recover recyclable glass from MBT. (C) 2011 Elsevier Ltd. All rights reserved.