We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs-type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by ? = 6.5¿107 &sgr;3.5 exp(-105(kcal/mol)/RT) where &sgr; (kbar) is the differential stress and ? (s-1) is the natural strain rate. For Mg2GeO4 spinel the flow law is ? = 2.6¿104 &sgr;2exp(-73 (kcal/mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to aproximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak 'decoupling' zone at the transition boundary in the mantle. Superplasticity brought about by transformation-induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.