Planar faults on (001) planes were formed during in-situ annealing and straining experiments on MoSi₂ single crystals in a high-voltage electron microscope. As-received and pre-deformed crystals grown by either the floatzone or the Czochralski technique were exposed to temperatures between 400 and 1200^°C. The faults are only formed if dislocations with 1/2 1111 Burgers vectors are present. The faults and the bordering partial dislocations were characterized by a transmission electron microscopy contrast analysis. The two partial dislocations bordering a fault have different types of Burgers vector. It is suggested that the faults result from a dissociation reaction according to 1/2[111] = 1/2[110] + 1/2[001], leading to intrinsic stacking faults on (001) planes. Probably, similar microstructural mechanisms control both the formation of the faults and the flow stress anomaly of the 111110 slip system in MoSi2.