Relaxation dynamics of salt-free, aqueous solutions of sodium poly(styrene sulfonate) (NaPSS) were investigated by mechanical rheometry and flow birefringence measurements. Two semidilute concentration regimes were studied in detail for a range of polymer molecular weights. At solution concentrations c < 10 mg mt, limiting shear viscosity eta(0) was found to scale with molecular weight and concentration as eta(0) similar to c(0.5)M(w) over nearly two decades in concentration. At higher solution concentrations, c > 10 mg mt, a change in viscosity scaling was observed eta(0) similar to c(1.5)M(w)(2.9), consistent with a change from simple Rouse dynamics for unentangled polyions to near-perfect reptation dynamics for entangled chains. Characteristic relaxation times tau deduced from shear stress and birefringence relaxation measurements following start-up of steady shearing at high rates reveal very different physics. For c < 10 mg mi,, both methods yield tau similar to c(-0.42)M(w)(-0.9). and tau similar to c(0)M(w)(0) for c > 10 mg mi,. Curiously, the concentration scalings seen in both regimes are consistent with theoretical expectations for salt-free polyelectrolyte solutions undergoing Rouse and reptation dynamics, respectively, but the molecular weight scalings are not. Based on earlier light scattering studies using salt-free NaPSS solutions, we contend that the unusual relaxation behavior is likely due to aggregation and/or coupled polyion diffusion. Simultaneous stress and birefringence measurements suggest that in concentrated solution, NaPSS aggregates are likely well permeated by solvent, supporting a loose collective of aggregated chains rather than the dense polymer aggregates previously supposed. Nonetheless, polyion aggregates of either variety cannot account for the inverse dependence of relaxation time on polymer molecular weight for c < 10 mg mt.