Cycloisomerization is a totally atom economic procedure which converts dienes and enynes into cyclic molecules. Modification of Grubbs' 2nd generation catalysts by reaction with dimethylformamide provides a new species able to catalyse this transformation. Selection of suitable conditions allowed high yields and selectivity. Studies performed in order to identify the catalytic species point to a non-carbenic ruthenium complex that has lost the phosphine. No hydride signals appeared. In addition, the reaction works with enynes and the new species catalyses efficiently crossed cyclotrimerizations of alkynes with diynes.