A method of optimisation of new extractants structure using the desirable function has been developed. Earlier the desirable function has been proposed by Harrington (Ind Qual Control 21: 494-498, 1965) for the optimisation of processes with several response functions. The developed method of optimisation of new extractants structure has been used for construction of phenolic type extractants (PTE) (a class of N-(2-hydroxy-5-nonylbenzil)-dialkylamines). It has been offered to use the charge on the nitrogen atom, the heat of dissociation of phenolic group, the logarithm of distribution factor of extractant between water and octanol (computed data) and maximum permissible concentration of extractants in aqueous phase (MPC) of the o-replaced phenols (the literary data) as the controllable parameters, defining efficiency of extractants for molecular design of PTE. During optimisation of extractants structure the quantity of alkyl substitutes at nitrogen atom, the carbon atoms number in these substitutes and the electronegative substitutes in o-position to phenolic group have been varied. As the result of the molecular design, the optimal structure of PTE found is N-(2,3-dihydroxy-5-nonylbenzil)-didecylamine, which perfectly meets the requirements to industrial extractants.