We investigated the deformational behavior of fine-grained calcite-anhydrite aggregates at conditions where the end-member phases are of similar strength and deform by dislocation creep and/or diffusion creep. Samples of various volume proportions were made by hot isostatically pressing CaCO3 and CaSO4 powders into dense aggregates. Grain sizes are 5 and 8 μm for two different anhydrites (An) and 8 μm for calcite (Cc). In all mixtures, grain sizes are 2--4 μm for both phases. The stress difference &sgr;, strain rate &egr;˙ and grain size d data are fit to a power law with &egr;˙∝&sgr;n. At 500 ¿C, grain flattening accounts for most of the sample strain for all mixtures, suggesting that dislocation creep is the dominant deformation mechanism. However, the mixtures, especially the 50:50, are often weaker than the end-member phases. At 600 ¿C the mixtures deformed dominantly by diffusion creep, as evidenced by n=1, the sensitivity of aggregate strength to the grain size of at least one component, little evidence is found for grain flattening and change in microstructures compared to the undeformed materials, and the mixtures are generally weaker than both end-members. However, the grain size difference is not enough to account for the strength difference. For a small number of experiments, the variability in the strengths appears to correlate with the fraction of interphase boundaries; weaker samples have higher fractions of boundaries between unlike phases (Cc-An) than stronger samples. We suggest that the relative weakness of the mixtures is at least in part due to enhanced boundary diffusion rates between unlike phases relative to like phases. ¿ 1999 American Geophysical Union