This study investigates in-situ catalysis during fast co-pyrolysis of wood and plastic and focus on gases from the initial reactions. The materials were prepared through melt-mixing and include birch wood together with low-density polyethylene (LDPE), polypropylene (PP) or polystyrene (PS). The co-pyrolysis experiments were conducted at 600°C with a residence time of 5 seconds. Introduction of CaCO3 resulted in increased yields of the gaseous compounds. Notably, the amount of gaseous compounds increased from 14 to 27 wt% for Wood-LDPE, from 17 to 31 wt% for Wood-PP, and from 44 to 81 wt% for Wood-PS. Particularly, compounds with a carbon number below 10 showed the most significant increase. This study contributes to a better understanding of the initial reactions of fast co-pyrolysis of wood and plastic. Specifically, it includes the impact of in-situ inorganic catalysts.