In situ catalytic co-pyrolysis (CCP) of mixtures of lignocellulosic biomass (LB) and plastic (polypropylene (PP) and polystyrene (PS)) with spent FCC catalyst (10% w/w loading) was performed in a semibatch reactor. The experiments were carried out using 30 g of feed at 510 degrees C and three different feed ratios of LB to plastic (1:2, 1:1, and 2:1). The effect of feed ratio on bio-oil yield, composition, and carbon yield of products (mainly, aromatics and olefins) was evaluated. A maximum aromatic selectivity of similar to 49%C with PP and similar to 82%C with PS was observed at a feed ratio of 0.5. Co-processing LB with PP produced more valuable chemicals like toluene and xylene at equal mixing ratios. The total yield of monoaromatics and olefins was maximum (similar to 73%C) at an LB to PP ratio of 0.5. When LB was co-fed with PS, the more valuable styrene monomers and ethylbenzene were found significantly in the product at intermediate mixing ratios. While co-feeding PP with LB improved the (H/C)(eff) ratio of the pyrolysis liquid, the change in the (H/C)eff ratio was insignificant in the case of biomass and PS mixtures. The spent FCC catalyst helps in oxygen removal from the bio-oil and improves its quality (higher aromatic fraction) compared to oil obtained from thermal co-pyrolysis of the biomass and plastic mixtures.