The structure of the t-butyldiazopyruvate (1), an intermediate in the synthesis of DPTA-like ligands (DTPA = diethylenetriaminepentaacetic acid), has been determined by X-ray analysis. The molecule, which lies in a crystallographic mirror plane, is characterised by a cis arrangement of the two carbonylic oxygen atoms. Molecular mechanics and quantum mechanical molecular orbital calculations, at EHMO and ab initio levels, were performed in order to investigate the conformational properties of the isolated molecule. The packing effect has been examined by means of the program PROMET (5) by calculating the packing energies for a number of possible crystal structures. It is found that the energetic differences calculated for the various effects are small and comparable. Thus, the cis conformation found in the experimental structure is not in deep contrast with the theoretical calculations for the free molecule which favor the trans conformer. An energy barrier is detected by all of the computational methods when two CO functional groups lie in orthogonal planes. Importantly, a search in the Cambridge Structural Database indicates that all of the torsional conformations are possible in the 0–180° range. The complexity of the contrasting energetic factors is presented in some detail.