The cationic actinide complex [(Et2N)3U]-[BPh4] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly toward the geminal dimer, but for bulky alkyne substituents the unexpected cis dimer is also obtained. Mechanistic NMR studies show that a η2-π-complex is formed between a terminal alkyne and an uranium-acetylide intermediate. This latter complex has been characterized spectroscopically. A plausible mechanistic scenario is proposed for the dimerization process.