Examining the effect of nonhydrostaticity on the shape of x-ray powder diffraction lines shows that a uniaxial stress field, as is generated in opposed-anvil type high-pressure apparatuses, can result in lines being split or having an asymmetric shape. The distribution of local stresses, generated by mismatches in shapes of neighboring grains within the specimen, results in orientation-dependent broadening: diffraction linewidths are generally proportional to1/E(hkl),whereE(hkl)is Young’s modulus for planehkl.Since anomalous diffraction patterns can be misinterpreted as indicating a phase transformation, the occurrence of new phases under nonhydrostatic pressure should be carefully confirmed. ©1997 American Institute of Physics.