The objective of this study was to determine the effect of inclusion of polyethylene glycol (PEG 8000) during in vitro incubation on gas production kinetics, organic matter digestibility (OMD) and the metabolisable energy (ME) content of foliage from the tannin containing tree species, Pistica lentiscus, Arbutus andrachne and Juniperm communis. The amount of gas produced when the foliage was incubated with buffered rumen fluid, was determined after 0, 3, 6, 12, 24, 48, 72 and 96 h of incubation in the presence of PEG at inclusions rates of 15, 30, 60 and 90 mg and in the absence of PEG. Their kinetics were described using the equation p = a + b (1-e-ct). Addition of PEG resulted in an increased gas production at almost all incubation times in all tree species. However species showed variable responses. After 3 h of incubation the PEG addition showed no significant effect on gas production when the foliage from A. andrachne was incubated, but had a significant effect on gas production as duration of incubation extended. The increase in gas production in response to increased levels of PEG inclusion was linear for P. lentiscus and J. communis. However, when the PEG inclusion rates exceeded 60 mg there was no significant increase in gas production when A. andrachne was incubated. The estimated parameters such as gas production rate(c) and gas production (a) from the immediately soluble fraction were not affected by the level PEG treatment, except that PEG addition at 90 mg had a significant effect on the gas production (a) from immediately soluble fraction of leaves of J. communis. Gas production (b) from the insoluble fraction (mL) and potential gas production (a+b), OMD and ME of tree leaves increased significantly with increasing levels of PEG addition. However, when PEG inclusion exceeded 60 mg these parameters showed no significant increase when leaves from A. andrachne were incubated. Although the mean increase in OMD per mg PEG supplementation was 0.131 digestibility units, the increase in ME per mg PEG supplementation was 0.0201 ME units. The elevated levels of gas produced, and increased OMD and ME estimates with the inclusion of PEG demonstrated the negative effect of tannins in foliage on digestibility. © South African Society for Animal Science.