This work uses rheology to probe the solid-like network formed in organically modified montmorillonite clay dispersions. All dispersions are based upon two commercial organoclays, Cloisite (R) 15A and 20A, which differ only in the quantity of surfactant used in the cation exchange reaction performed to render the natural Na-montmorillonite organophilic. In both cases, the amount of surfactant used in the cation exchange reactions is in excess of the cation exchange capacity of montmorillonite. Removal of unexchanged, free surfactant and subsequent thermogravimetric analysis demonstrates that the as-received, unexchanged, and surfactant extracted organoclays possess different levels of free surfactant and grafted surfactant surface coverage. The four resulting organoclays were dispersed in p-xylene at two concentrations and subjected to oscillatory and steady shear experiments in a controlled-stress rheometer. Experimental data show that all dispersions exhibit a solid-like response at low applied stress followed by yielding above an apparent yield stress. The presence of free surfactant is shown to significantly weaken the organoclay network, while a modest increase in surfactant surface coverage leads to a considerably more solid-like dispersion. The influence of free surfactant and surfactant surface coverage on organoclay exfoliation, tactoid size, and tactoid interactions are discussed. (c) 2006 The Society of Rheology.