Abstract The reaction of hydrazine with suitably substituted isoureas gives tetraazabicyclo[3.3.0]octanes. The products obtained in this reaction are extremely susceptible to the nature of the substrate and the reaction conditions, but a suitable combination of these can usually be found in order to form the bicyclooctane. In cases where the bicyclic system is not formed 1,2,4-triazoles are usually obtained but, occasionally, the imidazolone is the reaction product. A mechanistic sequence is proposed based on the variation and interconversion of the products formed in these latter reactions. The preference for addition of methylhydrazine at the substituted nitrogen is significantly reduced by the introduction of steric factors.