Novel 18- and 20-membered P,N-macrocycles have been obtained stereoselectively by covalent self-assembly of α,ω-bisphosphines, formaldehyde and benzylamine. Alternating SSSS/RRRR or RSSR diastereomers were formed in the row of the 14-, 16-, 18- and 20-membered macrocyclic aminomethylphosphines. For the first time it was demonstrated that the stereochemical result of the reaction depends on the even or odd number of the methylene groups between the two chiral phosphorus atoms in the initial α,ω-bisphosphines.