The reaction of several amino-substituted mono- and dichlorophosphanes (R2N)nPCl3–n (n = 2, R = i-Pr; n = 1, R = i-Pr, Cy) with GaCl3 to form phosphenium cations [(i-Pr2N)2P]+ (13+) and [(R2N)PCl]+ (14a, b+) has been investigated. The isolation and fully characterization of the first amino- and chloro-substituted phosphenium cation 14b[GaCl4] was accomplished. The insertion of these phosphenium ions into P–P bonds of dissolved P4 was examined. Amino- and chloro-substituted phosphenium cations 14a, b+ readily form the corresponding P5+-cages 15a, b+. No reaction was observed for the less electrophilic diamino-substituted phosphenium cation 13+. The molecular structures of [14b][GaCl4] and [15b][Ga2Cl7] are presented and, furthermore, the intriguing NMR spectra of the P5+-cations 15a, b+ are discussed.