beta-Irradiation (“e-beaming”) or swelling in ionic liquids, either process on its own, are common pretreatments for biomass in biorefinery scenarios. A combination of both, as it occurs for instance in beta-irradiation of biomass that has been insufficiently washed and still contains traces of ionic liquids, causes covalent derivatization of the cellulose and covalent incorporation of nitrogen, which can only originate from the IL traces. The nitrogen uptake occurred only in the presence of the ionic liquid and correlated linearly with both the irradiation dose and the concentration of the contained ionic liquid. The presence of other wood constituents (lignin, hemicelluloses) during beta-irradiation decreased nitrogen uptake, but did not prevent it. The derivatization of cellulose did not depend on the degree of crystallinity, but appeared to depend on the content of oxidized groups (carbonyl functionalities), also with a linear correlation. These initial observations prompted us to take a closer look at beta-irradiation as a way of “doing chemistry with physics”. We investigated the effect of beta-irradiation on cellulose alone, on pure 1,3-dialkylimidazolium ionic liquids, and on these ionic liquids when they contain dissolved model compounds or cellulose, and encountered some unexpected, rich chemistry. The talk describes the “beta-irradiation plus ionic liquid” system as a recent and surprising aspect of biorefinery research, presents the steps that led to a clarification of the underlying reaction mechanism, and addresses the possible potential of e-beaming in the presence of imidazolium ionic liquids for cellulose chemistry.