The initial stages of ErSi 2 formation on Si(001) are investigated by scanning tunneling microscopy and low energy electron diffraction. At sub-monolayer coverages two Er-induced superstructures are observed, where Er dimers are replacing the Si dimers. In the c(4x2) phase Er dimers alternate with Si dimers both along and perpendicular to the Si dimer row direction. The (2x3) phase, which evolves naturally from the c(4x2) phase with increasing coverage, consists of two adjacent chains of Er dimers and can be regarded as a precursor of the strain-field-controlled ErSi 2 nanowire growth.