In this paper, the transport and speciation of ruthenium under conditions simulating an air ingress accident was studied. Ruthenium dioxide was exposed to an oxidising environment at high temperatures (>1200 °C) in a tubular flow furnace. At these conditions, volatile ruthenium species were formed. A major part of the released ruthenium was deposited in the tube as RuO2. Depending on the experimental conditions, 12–35 wt. % of the released ruthenium was trapped in the outlet filter as RuO2 particles. At completely dry conditions using stainless steel tubes, only 0.1–0.2 wt. % of the released ruthenium reached the trapping bottle as gaseous RuO4. However, when alumina was applied as tube material or the atmosphere contained some water vapour and silver seed particles, the fraction of gaseous ruthenium reaching the trapping bottle increased to 5 wt. % which is close to thermodynamic equilibrium. This indicates that RuO2 does not catalyse the decomposition of RuO4.