Radical reactions have been studied between Me(3)SiPH(2) or (Me(3)Si)(2)PH and selected mono- and dialkenes, cyclohexene, 1-hexene, Ph(2)PCH=CH2, 1,5-octadiene, norbornadiene, and 1,4-pentadiene, a series chosen for their range of reactivities. The Me(3)SiPH(2)/alkene reactions [alkene in brackets] yield the following : [1-hexene], Me(3)SiP(H)(n-C6H13) (8), Me(3)SiP(n-C6H13)(2) (9); [Ph(2)PCH=CH2], (Ph(2)PCH(2)CH(2))(2)PSiMe(3) (12); [1,4-pentadiene], Me(3)SiP(CH2)(5) (14); [norbomadiene], Me(3)SiP(H)C7H9 (16A/16B). The (Me(3)Si)(2)PH/alkene reactons [alkene in brackets] yield the following : [Ph(2)PCH=CH2], Ph(2)PCH(2)CH(2)P(SiMe(3))(2) (18); [norbomadiene], (Me(3)Si)(2)PC7H9 (20). A new synthesis for (Me(3)Si)(2)PH using a LiAlH4/PH3 phosphinating reagent is described. Solvolysis of 8, 9, 12, 14, 16A/16B, 18, and 20 yields H2P(n=C6H13) (10), HP(n-C6H13)(2) (11), (Ph(2)PCH(2)CH(2))(2)PH (13), (CH2)(5)PH (15), H2PC7H9 (17), and Ph(2)PCH(2)CH(2)PH(2) (19). New compounds are characterized by spectral (P-31, C-13, and H-1 NMR, IR, and MS) data. Relative reactivities of phosphines in the (Me(3)Si)(3-n)PHn (n = 1-3) series toward alkenes decrease as PH3 > Me(3)SiPH(2) > (Me(3)Si)(2)PH. Origins of these reactivity differences are discussed.