We report on the synthesis of complex lead-free perovskite-type (1-x)(Ba0.8Ca0.2)TiO3-xBi(Mg0.75W0.25)O-3(BCT-xBMW) and (1-x)(Ba0.8Ca0.2)TiO3-xBi(Mg0.75Mo0.25)O-3(BCT-xBMM) solid solutions via conventional solid-state reaction route. The sintering temperature was adjusted as a function of compositionxto obtain dense samples (relative densities over 95%) at the same time minimizing bismuth evaporation. X-ray diffraction analysis shows the formation of single-phase perovskites for 0 <= x <= 0.10 in the BCT-xBMW series and increasing concentrations of impurity phases forx >= 0.15 and forx >= 0.05 in BCT-xBMM. A transition from a tetragonal to pseudo-cubic perovskite structure is observed in BCT-xBMW and BCT-xBMM atx = 0.05. The dielectric response has been characterized between -60 degrees C and 300 degrees C for BCT-xBMW, and between 30 degrees C and 300 degrees C for BCT-xBMM using impedance spectroscopy, showing a transition from ferroelectric to relaxor-like behavior atx >= 0.05. Additional polarization and Raman spectroscopy measurements reveal the occurrence of highly disordered systems. Analysis of the Raman spectra indicates structural phase changes and lattice modifications caused by chemical substitution. For the composition 0.8Ba(0.8)Ca(0.2)TiO(3)-0.2Bi(Mg0.75W0.25)O-3, a temperature-stable permittivity of about 600 (+/- 15% between -60 degrees C and 300 degrees C) and small losses of tan delta T <= 230 degrees C at 1 kHz are observed(,)making it a suitable dielectric material for high-temperature capacitors.