This invention provides a method of sterilising an S-nitrosothiol, for exampleS- nitrosoglutathione, without reductionof purity by more than about 5.0% through degradation. The invention allowssterile S-nitrosothiol or a sterile pharmaceuticalpre-composition comprising S-nitrosothiol, wherein the S-nitrosothiol is indry solid form, to be produced. The sterile pharmaceuticalpre-composition is mixed with one or more diluents, excipients, carriers,additional active agents, or any combination thereof, forexample sterile saline, to prepare a pharmaceutical composition of S-nitrosothiol for use.