Biomass pyrolysis performance is improved through co-pyrolysis with hydrogen rich plastic producing more liquid fuel products of higher quality. Furthermore, beyond the synergetic effect, in catalytic pyrolysis the presence of plastic may facilitate the contact between catalyst and biomass. In this study, the effect of pretreatment of plastic/biomass was investigated, in order to further improve the above aims. The first pretreatment method was pre-degradation treatment at relatively low temperatures where solid state reactions alter the structure of plastic/biomass but no volatile products are formed. In addition, a second pretreatment method was used for the catalytic pyrolysis of cellulose, namely co-pressing of cellulose and catalyst particles into mixed pellets. Thermogravimetric results confirmed that pre-degradation treatment reduced the decomposition temperature of cellulose. Pre-degradation diminished the hindering effect of the cellulose-derived char/catalyst-coke demonstrating significant interaction between cellulose and lldPE (linear low density polyethylene) before the charring/coking stage of cellulose. While in the absence of catalyst, there is a minimal interaction between lldPE and cellulose, as the molten lldPE layers around cellulose particles inhibited the escape of the biomass-derived volatile products, the use of catalyst along with pre-degradation provided maximum interaction between lldPE and cellulose pyrolysates which proceeded at lower reaction temperature without any hindrance from char/coke formation. Furthermore, pre-degradation treatment increased the liquid yield with associated increase in char/coke yield. The presence of lldPE during the cellulose pyrolysis decreased the concentration of char/coke on the catalyst while higher amounts of hard coke/char had accumulated on the pre-treated samples. Liquid product characterisation indicated that co-pressing of cellulose with catalyst is more effective in converting cellulose-derived products into aromatics. The results suggest that pre-treatment processes have increased the synergy between lldPE and cellulose, thereby enhancing the quantity and quality of the desired liquid product. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.