The electrical properties of polyaniline changes by orders of magnitude upon exposure to analytes such as acids or bases, making it a useful material for detection of these analytes in the gas phase. The objectives of this lab are to synthesize different diameter polyaniline nanofibers and compare them as sensor materials. In this experiment polyaniline nanofibers are synthesized using a two-phase interfacial polymerization method that yields nanofibers with relatively narrow diameter distributions centered around 30, 50, and 120 nm. The sensors are then fabricated by drop-casting aqueous dispersions of nanofibers onto electrode arrays to form films and measuring their change in resistance upon exposure to acids or bases. The sensor response is dependent on the surface area, diameter, and porosity of the nanofiber films. The larger diameter nanofibers have slower response times because of the difficulty for gas to diffuse through more material. The advantages to this lab include simplicity and low cost, making it suitable for both high school and college students, particularly in departments with modest means. (Contains 1 scheme and 1 figure.)