Using a synthetic oligopeptide, n-FKFEFKFEFKFE-c (KFE12), representative of a class of peptides that can undergo self-assembly into a three-dimensional matrix biomaterial, we show that the self-assembly occurs when solution conditions reduce intermolecular electrical double-layer repulsion below van der Waals attraction in accord with DLVO theory. This theory predicts that a critical coagulation concentration of counterions should be required to allow assembly and that this concentration should be inversely proportional to the valence of the counterion raised to the sixth power. Our experimental results show that KFE12, at low pH, exhibits critical coagulation concentrations in each of three different salt solutions, KCl, K2SO4, and K3Fe(CN)6, and that the relative values of these critical concentrations follow the predicted dependence upon anion valence. The theory further predicts that self-assembly should occur when the oligopeptide is electrically neutral even in the absence of exogenous salt. Our experimental results show that KFE12 indeed forms gels when neutralized with NaOH. Thus, we have gained fundamental theoretical understanding of how to control the assembly of this class of oligopeptide-based biomaterials.