A process of preparing cyclopentadiene or substituted cyclopentadienes, such as tetramethylcyclopentadiene. The process involves contacting under reaction conditions cyclopentenol or a substituted cyclopentenol with a catalyst selected from (1) crystalline and amorphous aluminum phosphates, (2) aluminophosphate molecular sieves, and (3) silicoaluminophosphate molecular sieves. The process produces cyclopentadienes in high isolated yields.