A theoretical study of SiH4 activation by CP(2)LnH complexes for the entire series of lanthanides has been carried out at the DFT-B3PW91 level of theory. The reaction paths corresponding to H/H exchange and silylation, formation of Cp(2)Ln(SiH3), have been computed. They both occur via a single-step sigma-bond metathesis mechanism. For the athermal H/H exchange reaction, the calculated activation barrier averages 1.8 kcal(.)mol(-1) relative to the precursor adduct Cp(2)LnH(eta(2)-SiH4) for all lanthanide elements. The silylation path is slightly exogenic (DeltaE similar to -6.5 kcal(.)mol(-1)) with an activation barrier averaging 5.2 kcal(.)mol(-1) relative to the precursor adduct where SiH4 is bonded by two Si-H bonds. Both pathways are therefore thermally accessible. The H/H exchange path is calculated to be kinetically more favorable whereas the silylation reaction is thermodynamically preferred. The reactivity of this familly of lanthanide complexes with SiH4 contrasts strongly with that obtained previously with CH4. The considerably lower activation barrier for silylation relative to methylation is attributed to the ability of Si to become hypervalent.