Amorphous ceramics with the chemical composition Si3B3N7 and SiBN3C were produced from single-source molecular precursors by polymerization and pyrolysis. The powder and fiber materials were investigated by means of energy filtering transmission electron microscopy. The intensity of elastically scattered electrons is recorded to calculate the pair distribution function of these ceramics. In the pair distribution function of Si3B3N7 three significant maxima at 0.144, 0.172, and 0.291 nm are clearly resolved and are assigned to the pair distances B–N, Si–N, and Si–Si (N–N), respectively, by comparison to crystalline materials. The predominant structural units of the ceramic are trigonal planar BN3 and tetrahedral SiN4 groups, which are close to their regular symmetry. The overall pair distribution function of SiBN3C is very similar to that of Si3B3N7; however, the maxima are broadened due to the incorporation of carbon into the network. High-resolution mapping of the elements Si, B, N, and C with electron spectroscopic imaging reveals a homogeneous distribution on a subnanometer scale without precipitation or separation of, for example, carbon-rich clusters. Similarly, elemental mapping of Si3B3N7 reveals a random distribution of the elements Si, B, and N at the same scale. Both new ceramics consist of an amorphous network with bonds and coordinations as preformed in the precursor.