Abstract Synthesis and structure of the two isomeric furoxancarboxylic acids 3 and 14 and of a few of their derivatives 10–13, 15, 16 is reported. 1, 3 Dipolar cycloaddition reactions of the product obtained by thermal decomposition of 4-phenyl-3-furoxancarboxylic acid and thermal equilibration of furoxan derivatives are also discussed.