C40 structured Mo(Si,Al)2 is a state-of-the-art material for high-temperature (1200-1600 C) electric heating systems, which can be used for electrification of industrial heating processes. However, the use is limited by a high creep rate and low flow stress at elevated temperature. Quaternary transition metal alloying of Mo(Si,Al)2 is relatively unexplored but holds a great promise as some transition metal disilicides (e.g. NbSi2 and VSi2) have shown anomalous strengthening at high temperature. We have investigated the microstructure of (Mo(1-y),X(y))(Si,Al)2, (X = Nb, V, Ta, Ti, Cr and 0.03<y<0.1) alloys, and show that the solubility of the transition elements in the C40 is very low, typically below 1.5 at.%. Instead, most of the alloying additions partition to the minority Mo5Si3 phase, which in unalloyed Mo(Si,Al)2 material has a tetragonal D8m structure. In some of the alloyed materials, a hexagonal D88 structure, along with D8m, has been shown to be present.