A few N,N’- diarylethylenediamines have been prepared, and decomposed with hydrogen bromide at elevated temperatures. An examination of the products has shown that the expected fused-ring heterocyclic compounds, e.g. indoline, are not formed; instead the main products are the corresponding 1,4-diphenylpiperazines and primary arylamines. Small quantities of other compounds have also been found. The significance of the various products has been discussed, and the total relevant evidence used in proposing a reaction scheme. The absence of fused-ring heterocyclics has been explained on steric and electronic grounds, and a comparison with syntheses of five-membered fused-ring compounds made.