Methods of marking plastic-based products and marked plastic-based products are provided. Some methods include irradiating the product to alter the functionalization of the plastic. In general, the present disclosure features methods of marking substrates, e.g., substrates including plastics, such as plastic-based products, such as polymer banknotes. Such plastics can be rigid or flexible, e.g., elastomeric. Such plastics can be thermoplastic or thermosets. In some cases, the products are marked by irradiating plastic-based materials, e.g., sheet materials, under conditions that alter characteristics of the irradiated plastic.