In order to avoid durability problems caused by an inadequate consolidation of concrete, self-compacting concrete (SCC) has been developed. The mix design of SCC aims at balancing a minimum flowability allowing air bubbles to escape and a maximum flowability in order to avoid segregation. Because of the higher demands on mix design and additional requirements related to casting, SCC mixtures are in general more sensitive to small variations in its mix composition compared to conventional vibrated concrete. Besides improving the robustness of SCC with admixtures like Viscosity-Modifying Agents (VMAs), it is also important to find out why certain mixtures are more robust than others. This paper investigates the influence of the paste volume and the waterto- powder ratio (volumetric) on the robustness of fresh SCC mixtures. Nine SCC mixtures with a paste volume of 350, 375, and 400 l/m^ and a volumetric water-to-powder ratio of 0.75, 0.90, and 1.05 were subjected to a variation of ±8 l/m^ water. The robustness of the produced mixtures was quantified measuring the slump flow, V-funnel time, L-box ratio, and sieve stability.