Peptide amphiphile molecules (PAs) developed in our laboratory self-assemble from aqueous media into three-dimensional networks of bioactive nanofibers. Multiple non-covalent interactions promote assembly of the supramolecular nanofibers and ultimately determine the bulk physical properties of the macroscopic gels. In this study, we use oscillatory rheology, Fourier-transform infrared spectroscopy, and circular-dichroism spectroscopy to better understand the assembly mechanism of a typical PA molecule known as PA-1. Self-assembly of PA-1 is triggered by counterion screening and stabilized by van der Waals and hydrophobic forces, ionic bridging, and coordination and hydrogen bonding. The concentration, electronic structure, and hydration of counterions significantly influence self-assembly and gel mechanical properties.