Sterilisation apparatus arranged controllably to generate and emit hydroxyl radicals. The apparatus includes an applicator which receives RF or microwave energy, gas and water mist in a hydroxyl radical generating region. The impedance at the hydroxyl radical generating region is controlled to be high to promote creation of an ionisation discharge which in turn generates hydroxyl radicals when water mist is present. The applicator may be a coaxial assembly or waveguide. A dynamic tuning mechanism e.g. integrated in the applicator may control the impedance at the hydroxyl radical generating region. The mist and/or gas and/or energy delivery means may be integrated with each other.