In this experiment, we intend to study crystal-field excitations in mononuclear dysprosium and erbium Single-Molecule Magnets (SMMs) derived from a series of perfectly tetragonal lanthanide SMMs. In the derived systems intended for the proposed experiment, we have rationally lowered the symmetry of the crystal field around the lanthanide ion. From ac magnetic susceptibility experiments, the symmetry lowering is found to worsen the SMM behaviour for the dysprosium system but improve that of the erbium system. The anisotropy barrier responsible for the SMM behaviour is governed by the crystal-field splitting, and these findings therefore indicate that the symmetry lowering affects this splitting in strikingly different ways for dysprosium and erbium. In order to clarify this, we want to probe the crystal-field splitting of these new low-symmetry systems by means of inelastic neutron scattering. The discrimination between crystal-field excitations and phonons will be based, not only on Q- and T-dependencies of the excitations, but also on comparison with the isostructural diamagnetic yttrium compound.