Abstract: Storage stable aqueous acidic solutions, often having a pH of up to 1 containing an ester peracid and/or an acid peracid can be obtained by reacting a diester satisfying the general formula R1—O—CO—R2—CO—O—R3 in which R1 and R3 each represents a alkyl group containing from 1 to 4 carbon atoms which may the same of different and R2 represents an aliphatic alkylene group optionally unsaturated which may be linear or branched containing from 2 to 6 carbon atoms with aqueous hydrogen peroxide in the presence of an acid, such as sulphuric acid and permitting the compositions to progress towards equilibrium concentrations. By starting with a diester, perhydrolysis generates an ester peracid which is a particularly effective peracid. The process can be controlled to produce solutions containing a high peracid content and within a wide range of ratios of ester peracid to acid peracid.