The dimeric macrocyclophosphazane [{P(μ-NtBu)}2(μ-NtBu)]2 ( 1) was reacted with elemental selenium. An unexpected C–N cleavage reaction occurred producing P4(μ-NtBu)3(μ-NH)3Se4 ( 2). The C–N bond cleavage is driven by the high steric ring strain present within the ring.