We have investigated storage moduli of silicone gels containing barium titanate in the presence of dc electric fields. The gels containing barium titanate swollen by silicone oil showed a storage modulus reduction, i.e. negative electrotheological effect. Contrary, no negative electrorheological effect was observed in the unswollen gels and silicon gels without barium titanate. Swollen silicone gels and most of silicone/BaTiO3 gels obeyed empirical quadratic dependence in electric field strength. Although an apparent phase separation was not observed in the swollen gel, microscopic phase separation may occur due to the difference in electric conductivity between particles (similar to10(-10) S/cm) and silicone oil (10(-9) S/cm), as a result, the negative electrotheological effect appears. The effects of frequency, degree of swelling, and the field strength have been discussed. (C) 2004 Elsevier Ltd. All rights reserved.