RHF calcns. have been carried out for 1,3,5-cycloheptatriene, norcaradiene, 1H-azepine, benzenimine (azanorcaradiene), oxepin and benzene oxide (oxanorcaradiene) employing the STO-3G, 4-31G and 6-31G* basis sets. Theor. geometries, conformation and barriers to ring inversion have been obtained and compared with the available exptl. structural data. The cyclotrienes possess a boat conformation with a const. admix. of 22% chair character leading to a flattening of the triene part and p-electron delocalization typical for a planar polyene. The structural data suggest that cyclic delocalization of either 6p (homoaromaticity) or 8p electrons (antiaromaticity) is not present in the 3 cyclotrienes. The former possibility, however, cannot be excluded in the case of 3 norcaradienes.