Abstract: The disclosure provides an improved method for preparing monoalkylated cyclopentadiene species in high yield and selectivity. In the process, either a solution of dicyclopentadiene magnesium or a cyclopentadiene magnesium halide is reacted with an alkylating agent in the presence of a modifying agent to provide the monoalkylated product. In the process of the disclosure, only a mono-alkylated species is produced with no detectible amount of dialkylated product observed.