The shear rheology and shear-induced microstructure of poly(ethylene glycol) (PEG)-based suspensions of acicular precipitated calcium carbonate (PCC) particles of varying particle aspect ratio (nominal L/D ∼ 2, 4, 7) are reported. These anisotropic particle suspensions demonstrate both continuous and discontinuous reversible shear thickening with increasing applied shear rate or stress similar to that observed for suspensions of spherical colloidal particles. The critical volume fraction for the onset of discontinuous shear thickening decreases as the average particle aspect ratio is increased. However, the critical stress for shear thickening is found to be independent of particle anisotropy and volume fraction. Rather, it can be predicted based on the minor axis diameter of the particles and is found to agree with values for near hard-sphere suspensions. Small angle neutron scattering during shear flow (Rheo-SANS) demonstrates that long-axis particle alignment with the flow direction is maintained throughout the range of shear stresses investigated, including the shear thickening regimes for both continuously and discontinuously shear thickening PCC/PEG suspensions. Rheo-SANS and transient rheological experiments indicate that this reversible shear thickening is a consequence of lubrication hydrodynamic interactions and the formation of transient hydroclusters of flow-aligned particles. (C) 2005 The Society of Rheology.