The aim of the present study was to evaluate the effects of sun-drying, ensiling and addition of Polyethylene Glycol (PEG) on deactivation of tannins in the Pistachio By-products Hull (PBH). The chemical composition, protein quality and phenolic compounds of fresh pistachio by-products hull, sun-dried and ensiled for either 30 or 60 days were assessed. Organic Matter Digestibility (OMD) and metabolizable energy (ME), either with or without PEG addition were assessed. Moreover, protein quality as according to CNCPS was determined. Following ensiling Water Soluble Carbohydrates (WSC), NDFom, ADFom and protein fractions B3 and C decreased (P<0.05), while the content of protein fractions A and B1 were increased (P<0.05). The concentrations of NDFom, ADFom, lignin (sa), NDIN and ADIN in dried PBH were increased (P<0.05), although, the content of WSC and protein fraction B2 decreased (P<0.05). Ensiling for 60 days caused decrease in the condensed tannins, hydrolysable tannins and protein precipitable phenolics (P<0.05). Adding PEG to fresh, sun-dried and ensiled PBH, however, increased the ME and OMD (P<0.05). In conclusion, adding PEG to fresh, sun-dried and ensilaged PBH increased (P<0.05) ME and OMD demonstrating the positive influence of PEG, not evident in either mere sun-drying or mere ensiling, on inactivation of tannins in PBH.