The azides 2a-i undergo regiospecific [3+ 2]-cycloaddition with the phospha-alkyne 1 to form the l, 2, 3, 4-triazaphospholes 3a-i; in the case of the reaction with 2J, an additional silyl shift occurs resulting in the formation of 4j. The latter product can by hydrolyzed to 6 which has also been prepared independently from 1 and hydrazoic acid (7). Flash vacuum pyrolyses of the 1, 2, 3, 4-triazaphospholes 3c and 3e produce mixtures of the isomers 13a, b and 14a, ...