The invention is a process for manufacturing a chemical product which may be later identified and distinguished from similar chemical products, and comprises the steps of (i) forming a reaction mixture comprising chemical intermediate A, chemical intermediate B and marker chemical M (ii) providing the conditions which enable chemical intermediate A and chemical intermediate B to react together to form chemical product C, and for marker chemical M to react either with chemical intermediate A to form a marker product AM or with chemical product C to form a marker product CM (iii) after said reactions have taken place, separating a mixture of product C with product AM and/or a mixture of product C with marker product CM from the reaction mix tune wherein product AM and product CM are both distinguishable from product C by analysis. By using the method of the invention, the marked product is formed in situ and in a mixture with unmarked product.
