The acidolysis of 3-endo-azidomethylbicyclo[3.3.1]non-6-ene (3) with methanesulphonic acid gave 4-azahomoadamant-4-ene (5) which was also produced from 3-endo-hydroxymethylbicyclo[3.3.1]non-6-ene (6) on treatment with NaN3–MeSO3H. The formation of (5) was rationalized by a π route cyclization followed by acidolytic ring expansion of intermediate azides on the basis of acidolysis of dideuterio-derivatives