Compositions near the morphotropic phase boundary (MPB) of the BiScO3-Pb(Mg1/3Nb2/3)O-3-PbTiO3 ternary system showed high-temperature relaxor properties (1 kHz) of T-max similar to 250 degrees C and a permittivity maximum of similar to 16 000. Transmission electron microscopy (TEM) was used to investigate the nature of the microstructure under ambient conditions and with in situ heating studies for samples with different composition and electric field-temperature histories. A mottled domain structure was observed with diffraction contrast TEM techniques and associated with frozen in polar micro-domains. These observations are consistent with the low field permittivity temperature measurements made under various frequencies (100 Hz-100 kHz) with a freezing temperature, T-f similar to 160 degrees C. Field-cooled (FC) samples exhibited a macro-domain structure similar to normal ferroelectric behavior. On heating the FC samples to approximately the T-max and above, the domain contrast was no longer apparent. When subsequently cooled to room temperature conditions, a micro-domain structure was observed, similar to the zero FC samples. The results are discussed with respect to permittivity measurements and phenomenological mechanisms contributing to the dispersion in the permittivity below the Curie maximum.