Catalytic hydrosilation of 1-(trimethylsilyl)-1-buten-3-yne (1) with three kinds of hydrosilanes (HSiMePh2, HSiMe2Ph, and HSiEt3) in CDCl3 at 30 degrees C in the presence of a catalytic amount of RuHCl(CO)(PPh3)(3) (2) gave five types of reaction pro ducts: (1E,3E)-CH(SiR3)=CHCH=CHSiMe3 (3), R3SiCH2CH=CHCH2SiMe3 (4), R3SiCH=C=CHCH2SiMe3 (5), (1Z,3E)-CH(SiR3)=CHCH=CHSiMe3 (6), and R3SiC=CCH=CHSiMe3 (7). Detailed investigations on the stoichiometric reactions of intermediate ruthenium species provided definitive evidence for the catalytic mechanism comprised of two catalytic cycles, the Chalk-Harrod cycle A and the modified Chalk-Harrod cycle C, and their interconnecting processes B and D. Product 3 is formed by the insertion of 1 into the Ru-H bond of 2 followed by the reaction of the resulting terminal dienyl complex Ru(CH=CHCH=CHSiMe3)Cl(CO)(PPh3)(2) (8) with hydrosilane. The latter process regenerates 2 and the sequence of reactions proceeds catalytically (cycle A). The reaction of 8 with hydrosilane is accompanied by a side reaction giving Ru(SiR3)Cl(CO)(PPh3)(2) (9) and CH2=CHCH=CHSiMe3 (10), and the latter is further converted to 4 by hydrosilation (process B). Silyl complex 9 thus generated in the system is the key intermediate for catalytic cycle C. Thus the insertion of 1 into the Ru-SiR3 bond of 9 via a formal trans-addition process forms an internal dienylruthenium complex Ru[C(=CHSiR3)CH=CHSiMe3]Cl(CO)(PPh3)(2) (11), which reacts with hydrosilane to give 5 and 6 and to regenerate 9. A part of 11 also undergoes beta-hydrogen elimination to give a dehydrogenative silation product 8 and hydride complex 2. Complex 2 thus formed resumes catalytic cycle A (process D). The catalytic intermediates 8, 9, and 11 were identified by NMR spectroscopy and/or elemental analysis. Factors controlling the catalytic cycles are discussed on the basis of the experimental observations.