This study unveiled the synergistic effects of catalytic co-pyrolysis of the macroalgae (Enteromorpha prolifera) and waste plastics (HDPE) for enhanced biofuels production. Results revealed that catalytic co-pyrolysis of macroalgae (seaweeds) and waste plastics lowered greatly the high contents of the acids, oxygenates and nitrogen-containing compounds in the seaweed bio-oils, while the aromatics and light hydrocarbons contents of the bio-oils were significantly increased, owing to blending of seaweeds with co-feeding element of high hydrogen content. Results also unveiled that the existence of the co-pyrolysis synergy inhibited catalytic coking and reduced solid residues' formation. Also, the addition of HZSM-5 catalyst further improved the reaction activity, yields and selectivity of aliphatic hydrocarbons than aromatics, and reduced the activation energy without changing the reaction mechanisms. This significant reduction in activation energy can be easily observed in the results of non-isothermal kinetic analyses performed using the TGA data via five different methods: Friedman, FWO, Vyazovkin, KAS and DAEM methods, as a single kinetics-evaluation model lacks the flexibility to account for different types of materials without further modifications to reaction scheme. The FTIR-spectra of the catalytic co-pyrolysis oils were consistent with the results of the GC/MS analysis of the oils from catalytic-co-pyrolysis. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.