During an hypothetical severe accident on a Pressurized Water Reactor (PWR), Fission-Products (FPs) are released from the nuclear fuel and may reach the reactor containment building. Among the FPs, ruthenium is of particular interest due to its ability to form volatile oxide compounds in highly oxidizing conditions. In addition, Ru is a very hazardous compound because it is chemically toxic and also because of its radiotoxicity. The topic of ruthenium is examined in terms of nuclear safety issues. A review of the literature regarding ruthenium oxides properties, gaseous and aqueous chemistry is compiled. The study focuses on the ruthenium tetroxide (RuO4) which is highly reactive and volatile, and is the most likely gaseous chemical form under the conditions prevailing in the containment. The interactions between ruthenium oxides and containment surfaces, which could be of main importance in the overall Ru behaviour, are also discussed. Finally, an evaluation of the possible revolatilisation phenomena of ruthenium adsorbed on PWR containment surfaces, or dissolved in the sump, under super-oxidizing conditions (radiolysis), is also presented. In this case, ruthenium dioxide (RuO2) must also be considered. Knowledge of all these phenomena is required to accurately predict ruthenium behaviour, and to make best estimate assessment of the potential ruthenium source-term.