The viscosity of flocculated suspensions of rhombohedral and acicular (needle-like) αFe2O3 (nonmagnetic) and acicular γFe2O3 (magnetic) particles in ethylene glycol was measured over broad ranges of concentration at shear rates D from a few sec-1 up to 1340 sec-1. The intrinsic viscosity, [η], derived from the concentration dependence of the viscosity, depends linearly on D-γ, where γ is rm1 2 and rm1 3 for suspensions of the acicular and rhombohedral particles, respectively. Intrinsic viscosities are discussed in terms of φpf = 2.5/[η], where φpf is the volume fraction of particles in a floc. At a D of 5.3 sec-1, [η] is about 30 for suspensions of the nonmagnetic particles and is 80 for the magnetic particles. The intrinsic viscosity at infinite shear rate, which increases from 3.3 to 6.0 with the axis ratio of the particles, indicates that small stable flocs exist. From [η] at 5.3 sec-1 and results from a theoretical treatment of floc growth by Vold, the floc size and number of particles in a floc were estimated and are compared with results from sedimentation rates. The dependence of shear stress on D conforms to the Casson equation, which gives the yield stress and viscosity at infinite shear rate. Data are also presented on xylene suspensions of acicular αFe2O3 particles stabilized with a polymeric dispersant. © 1979.