Aminophosphoranes 2 and 4−9 with a cyclohexylamino substituent and ring sizes varying from five to eight have been synthesized by oxidative addition reactions of cyclic aminophosphites with diols or 1,2-diketones. The reactivities of these phosphoranes are compared with those of the corresponding cyclic aminophosphites. The difference in hydrolytic pathways between amino- and analogous phenoxyphosphoranes is discussed. X-ray structures of two sets of compounds, (a) (C6H11NH)P(OCH2CMe2CH2O) (1) and (C6H11NH)P(OCH2CMe2CH2O)(1,2-O2C6Cl4) (2) and (b) (C6H11NH)P{O-(t-Bu)2C6H2)2CH2} (3) and (C6H11NH)P{(O-(t-Bu)2C6H2)2CH2}(1,2-O2C6H4)·1/2Et2O (4·1/2Et2O) have been determined and geometrical parameters compared between the P(III) and the corresponding P(V) compounds. In 1, the six-membered ring has a chair conformation with the amino group axial; in 2, the six-membered ring is located apical-equatorial in a trigonal bipyramidal geometry and has a boat conformation. The eight-membered ring has a boat-chair conformation in 3, whereas the same ring has a tub conformation in 4.