Pb[Mg1/3Nb2/3]O-3 was gradually substituted by Bi[Mg2/3Nb1/3]O-3 (BiMN) up to 30 mol%, with an overall modification by a constant fraction of PbTiO3 (10 mol%). Monophasic perovskite powders could be prepared via the B-site precursor route. Ceramic samples of the system showed a typical relaxor behavior of frequency-dependent dielectric dispersion. Values of the maximum dielectric constant decreased substantially with increasing BiMN concentration, whereas corresponding temperatures changed only moderately.