The long-term prediction of the equilibrium concentration of α-isosaccharinic acid (α-ISA) in cement pore water is a crucial step in the assessment of the role of cellulose in the safety of a cementitious repository. The aim of the present contribution is to summarise recent efforts in identifying the most important processes leading to the formation or degradation of α-ISA and in predicting its most likely concentrations in cement pore water. The issues considered are the kinetics involved in the formation of α-ISA, reactions of α-ISA with dissolved or solid compounds that may lead to limitations of its pore water concentrations and the chemical stability of α-ISA in a heterogeneous alkaline environment. Some new results are presented showing that α-ISA is degraded to low-molecular weight organic compounds in the presence of oxygen, whereas such processes occur only to a minor extent under anaerobic conditions. It is concluded that the processes involved in the degradation of cellulose under alkaline conditions are not sufficiently understood to explain fully the observed concentrations of α-ISA in long-term experiments.