The reactions of internal and terminal alkynes with organoalanes containing Et, n-Pr, and i-Bu groups in the presence of Cp(2)ZrCl(2) and MeZrCp(2)Cl were investigated with the goal of clarifying mechanistic details of some representative cases. Three fundamentally different processes, i.e., (i) C-M bond addition without C-H activation in the alkyl group, (ii) cyclic C-M bond addition via C-fi activation, and (iii) hydrometalation, have been observed, and the courses of these reactions significantly depend on (i) the nature and number of alkyl groups in organoalanes, (ii) their amounts, and (iii) solvents. The reaction of alkynes with Et(3)Al in the presence of 0.1 equiv of Cp(2)ZrCl(2) in nonpolar solvents, e.g., hexanes, proceeds via C-H activation to give the corresponding aluminacyclopentenes. Investigation of the reaction of 5-decyne with 1-3 equiv of Et(3)Al and 1 equiv of Cp(2)ZrCl(2), which gave mono-, di-, or trideuterated (Z)-5-ethyl-5-decene as shown in Scheme 10, together with the previously reported structural study on the reaction of Et(3)Al with Cp(2)ZrCl(2) leading to the formation of well-characterized bimetallic species 9, 10, and 11 (Scheme 11), supports a catalytic cycle involving bimetallic species 10 and 18 (Scheme 15). In summary, this process requires a zirconocene derivative containing one Zr-bound Et group which is linked to Et(3)Al (but not to Et(2)AlCl) through a Cl bridge, i.e., 18, to produce 10 via beta C-H activation. In sharp contrast, the reaction of Et(2)-AlCl-Cp(2)ZrCl(2) as well as of (n-Pr)(2)AlCl-Cp(2)ZrCl(2) does not involve any C-H activation processes. It proceeds well in chlorinated hydrocarbon solvents, e.g., (CH2Cl)(2), but it is extremely sluggish in nonpolar solvents, e.g., hexanes. The reaction may well involve direct C-Al bond addition to alkynes, as suggested earlier for Zr-catalyzed Me-Al bond addition to alkynes, but a few other alternatives cannot be ruled out on the basis of the currently available data. The reaction of alkynes with (n-Pr)(3)Al-Cp(2)ZrCl(2) in nonpolar solvents proceeds partially via C-H activation and partially via hydrometalation. In contrast with the C-H activation process observed with Et(3)Al, that with (n-Pr)(3)Al is totally dominated by dimerization of alkynes to give aluminacyclopentadienes rather than aluminacyclopentenes, reflecting a previously established generalization that propene can be much more readily displaced from Zr by alkynes than ethylene. Hydrometalation is the exclusive process with (i-Bu)(3)Al-Cp(2)ZrCl(2). This hydrometalation reaction, however, reveals a few interesting complications. Alkyl-substituted internal alkynes give double bond migrated products in addition to the expected hydrometalation products. With terminal alkynes the reaction produces the expected hydrometalation products and the 1,1-dimetalloalkanes in comparable yields. Various other related reactions involving other alkynes, e.g., PhC=CPh n-OctC=CH, and PhC=CH, and other reagents, e.g., Et(3)Al-MeZrCp(2)Cl, Et(2)AlCl-MeZrCp(2)Cl, and (n-Pr)(3)Al-MeZrCp(2)Cl, were also studied.