Organolanthanide complexes of the general type Cp'(2)LnE(TMS)(2) (Cp' = eta (5)-Me5C5; Ln = La, Sm, Y, Lu; E = CH, N; TMS = SiMe3) serve as effective precatalysts for the rapid intramolecular hydrophosphination/cyclization of the phosphinoalkenes and phosphinoalkynes RHP(CH2)(n)CH=CH2 (R = Ph, H; n = 3, 4) and H2P(CH2)(n)C=C-Ph (n = 3, 4) to afford the corresponding heterocycles CH3CH(CH2)(n)PR and Ph(H)C=C(CH2)(n)PH, respectively. Kinetic and mechanistic data for these processes exhibit parallels to, as well as distinct differences from, organolanthanide-mediated intramolecular hydroamination/cyclizations. The turnover-limiting step of the present catalytic cycle is insertion of the carbon-carbon unsaturation into the Ln-P bond, followed by rapid protonolysis of the resulting Ln-C linkage. The rate law is first-order in [catalyst] and zero-order in [substrate] over approximately one half-life, with inhibition by heterocyclic product intruding at higher conversions. The catalyst resting state is likely a lanthanocene. phosphine-phosphido complex, and dimeric [CP'2YP(H)Ph](2) was isolated and cystallographically characterized. Lanthanide identity and ancillary ligand structure effects on rate and selectivity vary with substrate unsaturation: larger metal ions and more open ligand systems lead to higher turnover frequencies for phosphinoalkynes, and intermediate-sized metal ions with Cp'(2) ligands lead to maximum turnover frequencies for phosphinoalkenes. Diastereoselectivity patterns also vary with substrate, lanthanide ion, and ancillary ligands. Similarities and differences in hydrophosphination vis-a-vis analogous organolanthanide-mediated hydroamination are enumerated.