Impact ionization rates in GexSi1−x alloys and strained layers are expected to be strongly influenced by the threshold energies for ionization. We have calculated the threshold energies along the principal crystal axes from empirical pseudopotential band structures for Ge, Si, and GexSi1−x alloys (x=0.25, 0.5, 0.75). Both relaxed (cubic) and strained [tetragonal, grown on (001) cubic Si] alloys were examined. The ionization process with the lowest threshold energy in Si is different from that in Ge, for both electron‐ and hole‐initiated ionization. For the cubic alloys, the lowest thresholds are ‘‘Si‐like’’ for compositions up to approximately x=0.65. The effect of strain on the GexSi1−x alloys is to increase the ratio of the lowest hole threshold to the lowest electron threshold for all compositions, suggesting the possibility that GexSi1−x strained layer avalanche photodiodes may exhibit superior noise properties.