A method of rendering harmless chemical and/or biological warfare agents on a surface, comprising: (i) exposing the agents to an atmosphere comprising peroxide vapour, and causing the peroxide vapour to condense on the surface having the chemical and/or biological agents thereon (ii) after step (i) exposing the agents to an atmosphere comprising further peroxide vapour, and causing the further peroxide vapour to condense on the surface having the chemical and/or biological agents thereon and (iv) optionally repeating step (ii).
