2-Thiobarbituric acids 1 can be dimerized anodically in acceptable yields if the special ultrasonic electrolysis cell described here is used. In some cases two isomeric dimers 2 and 4 were isolated. 2 is of the same type as the corresponding barbituric acid dimers. The other dimer 4 has a completely different, tetracyclic structure characterized by NMR spectra and X-ray diffraction. The mechanism postulated for the formation of the tetracyclic dimers is supported by cyclovoltammetric measurements.