The isomerism of dimethyl 2,2'-(7,8-dichloro-1H-benzo[b][1,4]diazepine-2,4-(3H,5H)diylidene)diacetate (1a) and dimethyl 2,2'-(7,8-dichloro-3-methyl-1H-benzo[b][1,4]diazepine-2,4-(3H,5H)diylidene)diacetate (1b) was investigated by ¹H, ¹³C and ¹⁵N NMR spectroscopy. In CDCl₃ solution, inversion of the diazepine ring was observed, whereas in (D₆)DMSO and (D₇)DMF solution, besides the ring inversion, a partial cleavage of one chelate ring appeared connected with (E/Z) isomerisation about one of the exocyclic C=C bonds. Gibbs free energies (ΔG) and Free activation energies (ΔG^≠) were calculated based on B3PW91-SCRF/ZVP DFT computations. Agreement between NMR data and DFT computations was found.