Unambiguous Identification of Regioisomeric Tetrathiafulvalenes by Mass Spectrometry: Application to Dihalogeno Derivatives and the First Synthesis of 4,4'(5')-Dichlorotetrathiafulvalene.-By means of low-resolution mass spectrometry the unambiguous structural assignment of regioisomers in the tetrathiafulvalene series is possible. This indicates that 4,4'(5')-dichlorotetrathiafulvalene is not prepared by treatment of tetrathiafulvalenyllithium with NCS as previously reported. This compound is available from the ester (II). -(ANDREU, R.; BLESA, M. J.; GARIN, J.; LOPEZ, A.; ORDUNA, J.; SAVIRON, M.; J.