Nanofibers of the conducting polymer polyaniline (PANi) are known to be easily prepared by a “rapid mixing” technique, which suppresses uncontrolled polymerization. Further, their diameter and morphology are determined by the particular acid in which the synthesis is carried out. In this study, we show how the concentration of the acid can be used to select between two nanoscale morphologies. PANi nanomaterials synthesized in 1 M and 0.1 M organic (10-camphorsulfonic acid and 2-acrylamido-2-methyl-1-propanesulfonic acid) and inorganic acids (HCl and HNO₃) were imaged via transmission electron microscopy. In general, higher acid concentration results in long, straight, discrete nanofibers, whereas lower concentration appears to produce networks of twisted or fused nanofibers. UV-Vis and FTIR data suggests that synthesis in 0.1 M acid yields a mixture of oxidation states of PANi.