A synthetic, structural and conformational study of 1,4,5,8‐tetraazadecalin (3) and several of its new, substituted derivatives was performed. Mass‐spectrometric and NMR‐spectroscopic data were taken to indicate a trans‐fused double chair in most cases. These findings, juxtaposed to the notorious elusiveness of trans‐1,4,5,8‐tetraoxadecalin (2) were rationalized in terms of a “reverse anomeric effect”. Substituent effects were also assessed: substituents in the two angular positions or on all four nitrogens preclude trans fusion in favour of the isomeric cis‐ and/or biimidazolidin‐2‐yl structures. The cis‐geometry also prevails in systems were it is imposed by bridging substituents.