Phosphine/silane mixtures have been investigated by ion trap mass spectrometry, and reaction pathways together with rate constants of the main reactions are reported. Mechanisms of ion-molecule reactions have been elucidated by single and multiple isolation steps, and exact mass measurements have been performed by Fourier transform mass spectrometry. The SiHn+ (n = 0-3) ions react with phosphine to give SiPHn+ (n = 1-4) ions. These ions further react and yield SiP2Hn+ (n = 2-5) and Si2PHn+ (n = 3-7) ions, which, in turn, react following different pathways with silane or phosphine to give Si2P2Hn+ (n = 5, 6, 8) and Si3PHn+ (n = 5-7), respectively. Mixed SiPHn+ (n = 1-4) ions also originate from the PHn+ (n = 0, 1) phosphine primary ions, as well as from the P2Hn+ (n = 0-3) secondary ions of phosphine. Protonation of phosphine from several ionic precursors is a very common process and yields the stable phosphonium ion, PH4+. Trends in total abundances of tertiary SiP2Hn+ (n = 2-4) and Si2PHn+ (n = 3-7) ions as a function of reaction time for different PH3/SiH4 pressure ratios show that excess of silane favors the nucleation of mixed Si-P ions. The mechanism and energetics of the reaction of Si+ with PH3 have been investigated by ab initio calculations, and the most stable structure of the SiPH+ product, with a hydrogen bridge between silicon and phosphorus, has been identified.