Oxidising substances (5.1) and organic peroxides (5.2); pose fire risk by yielding oxygen.
Regulatory detail & full definition
IMDG Class 5 — oxidisers and organic peroxides. Class 5.1 covers oxidising substances such as ammonium nitrate, calcium hypochlorite, and concentrated nitric acid that contribute oxygen and accelerate combustion. Class 5.2 covers organic peroxides which are thermally unstable and may decompose self-accelerating; many require temperature-controlled stowage. Both sub-classes require segregation 'separated from' flammables (Classes 3 and 4) and stowage away from heat sources. The Code limits packaging types and concentrations strictly for high-energy oxidisers such as ammonium nitrate.