High-temperature-resistant ceramics in the system Mo-Si-O-C were prepared from polymethylsiloxanes loaded with MoSi2 filler. Irreversible structural rearrangement reactions in the polymer-derived Si-O-C matrix phase and at the filler-matrix interface during heat treatment were detected by in situ electrical resistance measurements and impedance spectroscopy. At low temperatures a large difference in thermal expansion between the polymer and the filler resulted in an increase of resistance from 0.05 to 560 Omega . cm when the temperature was raised from 20 degrees to 680 degreesC. An increase of filler particle contact distances resulted in a high positive temperature coefficient (PTC200 degreesC = 4.64 K-1). At high temperatures carburization reaction of the filler on the particle surface resulted in contact formation which gave rise to a resistance lower than 10(-3) Omega . cm above 1000 degreesC.