In this work, polycarboranosiloxanes ( M n = 25.5-31.3 kDa) with well-defined structures and a varying content of the carborane moiety (2-15 wt%) were obtained. TGA and DSC methods were used to study their thermal properties and the rheological behavior of melts was studied. It was found that as the content of carboranes in the structure of these polymers exceeds 9 wt%, the crystallization of the siloxane chain is suppressed. Data of rheological studies indicate that phase separation occurs in such hybrid carboranosiloxane systems. In this work, polycarboranosiloxanes with well-defined structures and a varying content of the carborane moiety (2-15 wt%) were obtained. Data of rheological studies indicate that phase separation occurs in such hybrid carboranosiloxane systems.