Primary and secondary phosphines are investigated as chain-transfer agents for organolanthanide-mediated olefin polymerization. Ethylene polymerizations were carried out with [Cp'(2)LnH](2) and Cp'(2)LnCH(SiMe(3))(2) (Cp' = eta(5)-Me(5)C(5); Ln = La, Sm, Y, Lu) precatalysts in the presence of dicyclohexyl-, diisobutyl-, diethyl-, diphenyl-, cyclohexyl-, and phenylphosphine. In the presence of secondary phosphines, high polymerization activities (up to 10(7) g of polymer/(mol of Ln.atm ethylene.h)) and narrow product polymer polydispersities are observed. For lanthanocene-mediated ethylene polymerizations, the phosphine chain-transfer efficiency correlates with the rate of Ln-CH(SiMe(3))(2) protonolysis by the same phosphines and follows the trend H(2)PPh >> H(2)PCy > HPPh(2) > HPEt(2) approximately HP(i)()Bu(2) > HPCy(2). Under the conditions investigated, dicyclohexylphosphine is not an efficient chain-transfer agent for Cp'(2)LaPCy(2)- and Cp'(2)YPCy(2)-mediated ethylene polymerizations. Diisobutylphosphine and diethylphosphine are efficient chain-transfer agents for Cp'(2)La-mediated polymerizations; however, phosphine chain transfer does not appear to be competitive with other chain-transfer pathways in Cp'(2)Y-mediated polymerizations involving diisobutylphosphine. Regardless of the lanthanide metal, diphenylphosphine is an efficient chain-transfer agent for ethylene polymerization. Polymerizations conducted in the presence of primary phosphines produce only low-molecular-weight products. Thus, Cp'(2)Y-mediated ethylene polymerizations conducted in the presence of phenylphosphine and cyclohexylphosphine produce low-molecular-weight phenylphosphine- and cyclohexylphosphine-capped oligomers, respectively. For Cp'(2)YPPh(2)-mediated ethylene polymerizations, a linear relationship is observed between M(n) and [diphenylphosphine](-)(1), consistent with a phosphine protonolytic chain-transfer mechanism.