Relaxor properties ofSr0.61−xBa0.39Nb2O6:Cex3(SBN61:Ce) are examined by dielectric spectroscopy and linear birefringence (LB) as a function of frequency and temperature before and after poling. Relaxor behavior with large polydispersivity is observed above the ferroelectric phase transition temperatureTc.BelowTca long-range ordered ferroelectric state with low polydispersivity prevails. An Ornstein-Zenike analysis ofLBdata suggests that pureSBN61belongs to the3DIsing universality class. Doping withCe3−ions, which seem to act as random fields, enhances the relaxor properties and drives the system towards the random-field Ising model (RFIM) universality class. ©2000 American Institute of Physics.