The ground-state structures of neutral, cationic, and anionic phosphorus clusters P-n, P-n(+), and P-n(-) (n = 3-15) have been calculated using the B3LYP/6-311+G* density functional method. The P-n(+) and P-n(-) (n = 3-15) clusters with odd n were found to be more stable than those with even n, and we provide a satisfactory explanation for such trends based on concepts of energy difference, ionization potential, electron affinity, and incremental binding energy. The result of odd/even alternations is in good accord with the relative intensities of cationic and anionic phosphorus clusters observed in mass spectrometric studies.