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Abstract Pure copper single crystals of fixed orientation, containing dispersions of spherical particles of SiO2, were tested in tension. The crystals deform by single slip until the tensile axis reaches a symmetrical orientation, while the particles remain undeformed. The characteristic shear stresses, and rates of work hardening, of the crystals depend on particle size, spacing and volume fraction, on the temperature and strain rate of the test, and on the shear strength of the matrix material. The effect of particles on the critical shear stress is described, and it is shown that solution hardening and particle hardening are additive at the critical shear stress. The transition from the three stages of work-hardening characteristic of pure copper single crystals to the parabolic curve characteristic of dispersion hardened copper is described by analysing the effect of particles on each stage. Particles introduce a near-parabolic ‘stage α’ which occupies more and more of the stress-strain curve as the volume fraction of particles is increased. These parabolic stress-strain curves are compared with those of pure copper crystals of polyslip orientations. Comparison of these results with theory is the subject of a second paper.