The synthesis of a series of methyl‐substituted 8‐phenyl‐1,8‐diazabicyclo[4.3.0]nonanes is described. The preferred conformations of these compounds have been assigned on the basis of the 2850‐2500 cm−1 region of their infrared spectra and of the geminal coupling constant of the C‐9 methylene group. Copyright © 1969 Journal of Heterocyclic Chemistry