Phenyldiazene, C6H5N=NH, the first monosubstituted diazene to be observed directly, exhibits unusual chemical properties. The reaction of phenyldiazene with itself is cleanly bimolecular over a concentration range of 650, and its disappearance in this way accounts for the failure to isolate pure phenyldiazene at room temperature. The major products of the bimolecular reaction are benzene (65-80 %) and nitrogen (80-84 %). Hydrogen transfer is important in the rate-limiting transition state as shown by kH/kD ≄ 4-5. The activation energy for the bimolecular reaction is low (~9 kcal/mole) and the activation entropy very negative (–23 eu), indicating a highly organized transition state. A mechanism for the bimolecular reaction involving formation of a cage radical pair is suggested. The nitrogens of the diazene group are nonnucleophilic toward methyl chloroformate and benzenesulfonyl chloride, as would be expected for azo compounds. Phenyldiazene is very reactive toward oxygen, reacts with 1,4-benzoquinone, hydroxide ion, and diazene (diimide), and is unreactive toward azobenzene.