Amination of [ClP(μ‐NtBu)]2 (1) using NH3 in THF gives the cyclophospha(III)zane dimer [H2NP(μ‐NtBu)]2 (2), in good yield. 31P NMR spectroscopic studies of the reaction of 1 with 2 in THF/Et3N show that almost quantitative formation of the cyclic tetramer [{P(μ‐NtBu)}2(μ‐NH)]4 (3) occurs. The remarkable selectivity of this reaction can (in part) be attributed to pre‐organisation of 1 and 2, which prefer cis arrangements in the solid state and solution. The macrocycle 3 can be isolated in yields of 58–67 % using various reaction scales. The isolation of the major by‐product of the reaction (ca. 0.5–1 % of samples of 3), the pentameric, host–guest complex [{P(μ‐NtBu)2}2(μ‐NH)]5(HCl)⋅2 THF] (4⋅2 THF), gives a strong indication of the mechanism involved. In situ 31P NMR spectroscopic studies support a stepwise condensation mechanism in which Cl− ions play an important role in templating and selection of 3 and 4. Amplification of the pentameric arrangement occurs in the presence of excess LiX (X=Cl, Br, I). In addition, the cyclisation reaction is solvent‐ and anion‐dependent. The X‐ray structures of 2 and 4⋅2 THF are reported.