Abstract: The present invention relates to a process for the preparation of a solution comprising a first peroxy acid comprising performic acid and a second peroxy acid, said process comprising forming a carboxylic acid solution comprising a first carboxylic acid comprising formic acid, a second carboxylic acid and hydrogen peroxide, wherein the amount of formic acid is from 0.5 to 20% by weight of the amount of the second carboxylic acid, and allowing the components to react to form a solution comprising performic acid and said second peroxy acid, the amount of peroxy acids being at least 5% by weight. The invention also relates to a storable solution comprising performic acid and said second peroxy acid. The solution can be used as a disinfecting agent for controlling micro-organisms.