Burford and co-workers recently (2007) investigated the synthesis, structure, and pseudorotation of a family of cyclotetraphosphinophosphonium ions comprehensively. We now report theoretical computations through the CCSD(T) level to study the conformations of the parent model rings, P5H6+ and P5Me6+. The all-trans conformations on the full pseudorotational cycle for the cyclotetraphosphinophosphonium cation (P5H6+) and for its methyl-substituted derivative (P5Me6+) were located systematically on their potential energy surfaces. The potential energy for the pseudorotational circuit of P5H6+ is smooth and monotonic with only one minimum (T-3(4)) and one maximum (T-4(3)), but the analogous P5Me6+ circuit has two minima. The P5H6+ and P5Me6+ potential surfaces appear to be qualitatively very different from that for the well investigated tetrahydrofuran. Since the T-3(4) form Of P5H6+ avoids unfavorable eclipsing PH-PH and lone pair-lone pair interactions, it is the global minimum among all possible P5H6+ configurations and conformations.