Chlorine is one of the most problematic elements in agriculture biomass. To better understand the behaviour of chlorine in the oat straw pyrolysis process, ammonium chloride was doped. The aim of this work is to determine the effect of inorganic chlorine addition to agricultural biomass on kinetics and thermodynamic parameters of the pyrolysis process. Preliminary experimental investigations included thermogravimetric (TG) and simultaneously mass spectrometry (MS) analysis to study kinetic parameters. The thermal analysis of raw oat straw and oat straw with doped ammonium chloride was carried out for four heating rates (5, 10, 20 and 30 K/min). On the basis of TGA data, the influence of ammonium chloride doping to straw and the degree of conversion on changes in activation energy, pre-exponential factor, enthalpy, Gibbs free energy, and entropy were determined. The doping of ammonium chloride to the straw increased the mean value of the activation energy. Additionally, the pyrolysis process of raw oat straw and oat straw with doped ammonium chloride was investigated in a fixed bed reactor under special system with pre-programmed heating curves for 873 K. The collected char was surface analyzed by the X-ray photoelectron spectroscopy (XPS) method to determine the forms of occurrence of elements such as carbon, nitrogen, and chlorine. Only organic forms of chlorine were identified in the char.