jats:titleAbstract</jats:title>jats:pTo study the mechanical behavior of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (jats:italicf</jats:italic>jats:subPer</jats:sub>) between 0.1 and 0.8. Each sample was deformed in torsion at jats:italicT</jats:italic> = 1523 K, jats:italicP</jats:italic> = 300 MPa at a constant strain rate to a final shear strain of jats:italicγ</jats:italic> = 6 to 7. The stress‐strain data and calculated values of the stress exponent, jats:italicn</jats:italic>, indicate that Ol in our samples deformed by dislocation‐accommodated sliding along grain interfaces while Per deformed via dislocation creep. At shear strains of jats:italicγ</jats:italic> < 1, the strengths of samples with jats:italicf</jats:italic>jats:subPer</jats:sub> > 0.5 match model predictions for both phases deforming at the same stress, the lower‐strength bound for two‐phase materials, while the strengths of samples with jats:italicf</jats:italic>jats:subPer</jats:sub> < 0.5 are greater than predicted by models for both phases deforming at the same strain rate, the upper‐strength bound. These observations suggest a transition from a weak‐phase supported to a strong‐phase supported regime with decreasing jats:italicf</jats:italic>jats:subPer</jats:sub>. Above jats:italicγ</jats:italic> = 4, however, the strength of all two‐phase samples is greater than those predicted by either the uniform‐stress or the uniform‐strain rate bound. We hypothesize that the high strengths in the Ol + Per system are due to the presence of phase boundaries in two‐phase samples, for which deformation is rate limited by dislocation motion along interfacial boundaries. This observation contrasts with the mechanical behavior of samples consisting of Ol + pyroxene, which are weaker, possibly due to impurities at phase boundaries.</jats:p>