The aim of the present work was to determine the chemical composition including condensed tannin contents of leaves of some trees used for small ruminant animals in Turkey, and examine their relationships with in vitro gas production parameters. Crude protein content ranged from 5.62 to 14.1%, with Morus alba having the highest content. The content of neutral (NDF) and acid detergent fiber (ADF) ranged from 42.3 to 56.9% and 28.3 to 34.2% respectively. The NDF content of Juniperus communis was significantly higher than the others. The ash content ranged from 4.99 to 15.8% with Morus alba having highest ash. Total condensed tannin (TCT) content of tree leaves ranged from 1.42 to 21.3% with Juniperus communis having the highest TCT, most of which was in the soluble form. Gas production at 96 h incubation for leaves of Quercus libari was significantly higher than the others. The rank order in terms of potential gas production was Quercus libari ≥ Populus nigra ≥ Morus alba > Juniperus communis. There was no significant difference among tree leaves in terms of the rate of gas production from insoluble but fermentable fraction. The gas volume of Quercus libari obtained from soluble fraction was significantly higher than the other tree species. The gas production from the insoluble but degradable fraction was significantly higher for Quercus libari than the other tree leaves whereas the potential gas production of Quercus libari was significantly higher than those of Morus alba and Juniperus communis. Total and soluble condensed tannins, NDF and ADF were negatively correlated with estimated parameters of gas production.