PX4+[Al(OR)4]- (X = I: 1a, X = Br: 1b) was prepd. from X2, PX3, and Ag[Al(OR)4] [R = C(CF3)3] in CH2Cl2 at -30 Deg in 69-86% yield. P2X5+ salts were prepd. from 2PX3 and Ag[Al(OR)4] in CH2Cl2 at -30 Deg yielding almost quant. P2X5+[Al(OR)4]- (X = I: 3a, X = Br: 3b). The P-rich P5X2+ salts arose from the reaction of cold (-78 Deg) mixts. of PX3, P4, and Ag[Al(OR)4] giving P5X2+[Al(OR)4]- (X = I: 4a, X = Br: 4b) with a C2v-sym. P5 cage. Silver salt metathesis presumably generated unstable PX2+ cations from PX3 and Ag[Al(OR)4] (X = Br, I) that acted as electrophilic carbene analogs and inserted into the X-X (P-X/P-P) bond of X2(PX3/P4) leading to the highly electrophilic and CH2Cl2-sol. PX4+ (P2X5+/P5X2+) salts. Reactions that aimed to synthesize P2I3+ from P2I4 and Ag[Al(OR)4] instead led to anion decompn. and the formation of P2I5(CS2)+[(RO)3Al-F-Al(OR)3]- (5). All salts were characterized by variable-temp. soln. NMR studies (3b also by 31P MAS NMR), Raman and/or IR spectroscopy as well as x-ray crystallog. (with the exception of 4a). The thermochem. vols. of the P-X cations are 121 (PBr4+), 161 (PI4+), 194 (P2Br5+), 271 (P2I5+), and 180 .ANG.3 (P5Br2+). The obsd. reactions were fully accounted for by thermochem. calcns. based on (RI-)MP2/TZVPP ab initio results and COSMO solvation enthalpy calcns. (CH2Cl2 soln.). The enthalpies of formation of the gaseous P-X cations were derived as +764 (PI4+), +617 (PBr4+), +749 (P2I5+), +579 (P2Br5+), +762 (P5I2+), and +705 kJ mol-1 (P5Br2+). The insertion of the intermediately prepd. carbene analog PX2+ cations into the resp. bonds were calcd., at the (RI-)MP2/TZVPP level, to be exergonic at 298 K in CH2Cl2 by DrG(CH2Cl2) = -133.5 (PI4+), -183.9 (PBr4+), -106.5 (P2I5+), -81.5 (P2Br5+), -113.2 (P5I2+), and -114.5 kJ mol-1 (P5Br2+). [on SciFinder (R)]