The aim of STEP is to improve by some five orders of magnitude the test of the validity of the Equivalence Principle (EP) of General Relativity or to detect new interactions possibly violating it. Such do occur in some attempts of unification of gravitation with quantum field theories. The procedures developed for M3 STEP (a possible ESA mission) for optimising the sensitivity by selecting materials as 'different' as possible are presented. Parameters like N + Z, N - Z as well as nuclear electrostatic energy density have been used. The experimental requirements for the highly sensitive differential accelerometers are also taken into account.