Dimeric cyclophosphazanes [{P(μ-NR)}2(μ-NR)]2 [R = (t)Bu ( ) and (i)Pr ( )] were oxidized with elemental selenium. During these reactions an unexpected C-N bond cleavage and N-H bond formation occurred. Compound produced P4(μ-N(t)Bu)3(μ-NH)3Se4 ( ) where three (t)Bu groups were lost in the form of isobutylene. In contrast, during the oxidation of the less sterically hindered , the resulting product, P4(μ-N(i)Pr)5(μ-NH)Se4 ( ), showed only one substituent loss. Theoretical studies confirmed the steric nature of the driving force underlying the different outcomes.