In order to measure directly the extent of elastic strain partitioning between the phases in a two phase material, room temperature uniaxial compression experiments have been performed on an olivine + magnesiow¿stite sample within the neutron beam at the ISIS neutron spallation source, Rutherford Appleton Laboratory, U.K. Neutron diffraction data were collected at fourteen different applied loads. At each load the olivine and magnesiow¿stite lattice parameters in directions parallel and normal to the direction of applied load, were extracted from the diffraction data, and the axial and radial elastic strains experienced by each phase were calculated from these parameters. The observed strain partitioning is in close agreement with that predicted by the widely used Hashin-Shtrikman analysis of the elastic properties of composites, confirming both the validity of that analysis, and the potential which the experimental technique has for providing insight into the controls on the mechanical properties of polymineralic materials. ¿ 2001 American Geophysical Union