We have determined the ground-statewave functions and crystal-field-level schemes of CeRh2Si2 and CeRu2Si2 using linear polarized soft x-ray-absorption spectroscopy (XAS) and inelastic neutron scattering. We find large crystal-field splittings and ground-state wave functions which are made of mainly J(z) = |+/- 5/2 > with some amount of |-/+ 3/2 > in both the compounds. The 4f(0) contribution to the ground state of several members of the CeM2Si2 family with M = (Cu, Ru, Rh, Pd, and Au) has been determined with XAS, and the comparison reveals a trend concerning the delocalization of the f electrons. Absolute numbers are extracted from scaling to results from hard x-ray photoelectron spectroscopy on CeRu2Si2 by Yano et al. [Phys. Rev. B 77, 035118 (2008)].