The conformational equilibria of 1,3,2-dioxaphosphorinanes 4-8, featuring diequatorial attachment of the ring to five-coordinate phosphorus, were perturbed by a series of substituents at C5 (Me, Ph, t-Bu). Unlike the analogous phosphoranes that feature equatorial/apical ring attachment and populate boat/twist conformations even when unbiased by ring substituents, no H-1 NMR evidence could be found for population of a nonchair conformation, even in the presence of an axial 5-t-Bu (8). The failure to readily form boat/twist conformations also is contradictory to their known ease of population by 1,3,2-dioxaphosphorinanes containing three- or four-coordinate phosphorus. Both steric and stereoelectronic rationales are offered for this highly significant but hitherto unrecognized conformational property of 1,3,2-dioxaphosphorinane rings attached diequatorially to five-coordinate phosphorus. Perturbation of a chair-chair equilibrium (A reversible arrow B) was observed that allowed the determination of conformational energies (A values, kcal/mol, in C6D6) for the groups at C5 : Me, 0.8; Ph, 1.6; t-Bu, 1.7. These values are similar to those for C5 substituents on 1,3-dioxane rings with the exception of 5-Ph (A value 1.0 kcal/mol for 1,3-dioxane). The similarity of the A values for the Ph and t-Bu substituents in the 1,3,2-dioxaphosphorinane ring is tentatively assigned to increased steric repulsions present when the 5-Ph is axial because of the apical P-O bond of the five-coordinate phosphorus.