For almost sixty years chemists have used &e reaction of isocyanidee with hydrazoic acid as the method of choice for the preparation of 1 -substituted tetracoles (I).' Y R?V-C-+ I-IN, -I Today isocyanides are readily and cheaply available'.and HN, solutions in inert solvents are It is possible that a minor pathway in the formation of I involves a direct reaction of RNC with HN,. Supporting this is the fact that the 5-aminotetrasole derivative (VIII 11%) and the expected tetrasole (VII 85%) are both obtained on treatment of the isocyanide (VI) with HN,. We suggest the following mechanisms or prototropic variants for the generation of VIII. EtOsCCHsNC + HN, Scheme 4 EtOaCCH&=C\ VI/ kC;x E-aCCH+ vH 2' HN Hp-N=fl J ,,/ H d -H. VH EtOaCCHaN 'N \ \N-rs 3 EtOaCCHsN=C=N-H e EtOsCCHaNH-C=N VIII IX X In support of this mechanism is the fact that: 1) 1,4-disubstituted tetrasolium salts cleave to carbodiimides on titration with triethylamine, "s 2) cyanamides (X) react with HN, to yield 5-aminotetrasoles as if they had the carbodiimide structure (IXX9 and 3) no aminotetrazole (VIII) is obtained when VI is treated with HN, in the presence of BF3.etberate. We wish to thank the U. S. Public Health Service for their support of this research (GM-13980) and for a predoctoral fellowship for D. M. Z.