This paper is concerned with a direct experimental and modeling comparison of the rheology of carbon black (CB) and multiwalled carbon nanotube (CNT) suspensions within a Newtonian epoxy matrix. Experimental observations of the effect of shear on CB and CNT microstructure are reported for a range of CB and CNT suspension concentrations. Steady shear, time dependent shear behavior, and oscillatory linear viscoelasticity (LVE) of the suspensions are reported and remarkably strong similarities were observed between the CB and CNT suspension rheology, for example, 4 wt. % CB and 0.4 wt. % CNT suspensions. Optical observations showed that both the CB and CNT microstructures were shear rate sensitive and a structure-dependent hybrid Maxwell-Voigt phenomenological model with a yield stress was developed that gave a reasonable fit to the rheological data. The structure model parameters for both systems were found to be of a similar order of magnitude, although the onset of rheology development for the two systems occurred with a decade difference of concentration. The experimental rheological results and model fit indicated that for both CB and CNT suspensions, the observed rheology changes were dominantly controlled by aggregate size and interaction, rather than the respective particulate and filament morphology of the CB and CNT microstructures. The critical concentration for the onset of rheology development was, however, dependant on the basic morphology of the CB and CNT suspensions. (C) 2012 The Society of Rheology. [http://dx.doi.org/10.1122/1.4751871]