The threshold switching properties of the amorphous semiconductor As/sub 15/Te/sub 70/Ge/sub 15/ and the organic semiconductor melanin have been examined with threshold switching experiments. Measurements of delay time, energy and average resistance as functions of applied voltage were made by a computer controlled data acquisition system. These measurements were made on As/sub 15/Te/sub 70/Ge/sub 15/ at 243, 273 and 298/sup 0/K. A time dependent electrothermal model was solved numerically. The material parameters that were measured for this model are activation energy, sample thickness, conductivity at T equal to infinity, and area of the electrodes. With these measured parameters the calculation was compared to the experimental values of delay time, energy, and average resistance as functions of applied voltage. The measured data and the calculations are in good agreement. Melanin was prepared from a lyophilized powder into a hydrated pellet. The melanin pellets were studied at 298, 190, and 77/sup 0/K with a double pulse apparatus. Time dependent current versus voltage characteristics were also measured. Melanin exhibits threshold switching at low electric fields (about 10/sup 2/ V/cm). The results from the melanin experiments show that threshold switching in melanin can be explained by a thermal runaway model. A pseudo-memory effect was also found in melanin.