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(SiMe3)(2) (Cp' = eta(5)-Me5C5; 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(SiMe3)(2) protonolysis by the same phosphines and follows the trend H2PPh >> H2PCy > HPPh2 > HPEt2 approximate to (HPBu2)-Bu-i > HPCy2. Under the conditions investigated, dicyclohexylphosphine is not an efficient chain-transfer agent for Cp'2LaPCy2- and Cp'2YPCy2-mediated ethylene polymerizations. Diisobutylphosphine and diethylphosphine are efficient chain-transfer agents for Cp'La-2-mediated polymerizations; however, phosphine chain transfer does not appear to be competitive with other chain-transfer pathways in Cp'Y-2-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'Y-2-mediated ethylene polymerizations conducted in the presence of phenylphosphine and cyclohexylphosphine produce low-molecular-weight phenylphosphine- and cyclohexylphosphine-capped oligomers, respectively. For Cp'2YPPh2-mediated ethylene polymerizations, a linear relationship is observed between M-n and [diphenylphosphine](-1), consistent with a phosphine protonolytic chain-transfer mechanism.