mCPBA can be prepared by reacting m-chlorobenzoyl chloride with a basic solution of hydrogen peroxide, followed by acidification.
It is sold commercially as a shelf-stable mixture that is less than 72% mCPBA, with the balance made up of m-chlorobenzoic acid (10%) and water. The peroxyacid can be purified by washing the commercial material with a  sodium hydroxide and potassium phosphate solution buffered at pH = 7.5. Peroxyacids are generally slightly less acidic than their carboxylic acid counterparts, so the acid impurity can be extracted if the pH is carefully controlled. The purified material is reasonably stable against decomposition if stored at low temperatures in a plastic container.
In reactions where the exact amount of mCPBA must be controlled, a sample can be titrated to determine the exact amount of active oxidant.