A series of polyelectrolytes with controlled molecular weight, a narrow chain-length distribution, and systematic structural differences were synthesized using atom-transfer radical polymerization and investigated as stabilizers for magnetite nanoparticles in aqueous suspensions. Structural differences include the degree of polymerization, the chain architecture, and the identity of the charged functional unit. The synthesized polymers are sulfonated poly(2-hydroxyethyl methacrylate), a block copolymer of the former with poly(n-butyl methacrylate), poly(sodium styrene sulfonate), poly(sodium acrylate), and poly(sodium vinylphosphonate). The colloidal stability is assessed by measuring the fraction of particles, based on turbidity, that sediment after a period of time at increasing ionic strength. Sedimentation results are complimented by dynamic light scattering determinations of the hydrodynamic diameter of the particles that remain suspended. When adsorption and sedimentation are conducted at high pH, poly(sodium acrylate) and poly(sodium vinylphosphonate) yield the most stable suspensions because of their strong coordinative interactions with the iron oxide surface. At low pH, the polymers that retain pendant negative charges (each of the sulfonated polymers) yield high stable fractions at all ionic strengths investigated up to 100 mM (NaCl), whereas polyelectrolytes that become protonated with decreasing pH, poly(sodium acrylate) and poly(sodium vinylphosphonate), lose their stabilizing capacity even at low ionic strengths. The chain-length distribution profoundly alters a polymer's stabilization tendencies. Two poly(sodium acrylate) samples with the same number-average molecular weight but widely different chain-length distributions proved to have opposite tendencies, with the polydisperse sample being a good stabilizer and the low polydispersity one being a strong flocculant. This investigation provides guidelines for the design of polymeric stabilizers for magnetite nanoparticles according to the pH and ionic strength of the intended application.