A new cyclodimerization of ruthenium carbenes and alkynes is reported. The cyclodimerization produces substituted cyclopentadienes and consumes the carbene complex. Cyclodimerization of alkynes is predominant at low alkene concentrations and was found to be competitive with enyne metathesis. The presence of the ortho alkyl ether in the ruthenium carbene complex appears to be a critical determinant for the formation of cyclopentadienes. At higher alkene concentration, the observed cyclodimerization can be suppressed, favoring enyne metathesis.