Product analysis has been carried out on the pyrolysis reaction in the liquid and vapor phase of 4- and 5-methyl-3-carbomethoxy-Δ2-pyrazolines and 3-methyl-3-carbomethoxy-Δ1-pyrazoline. The proportion of cyclopropane product was found to be greater than earlier reports had indicated and was higher for the vapor-phase pyrolysis than for the liquid-phase reaction. Vapor-phase pyrolysis of Δ2-pyrazolines was found to require a catalyst, such as potassium hydrogen phosphate, which is believed to make possible the transformation of the Δ2-pyrazoline into the pyrolyzable Δ1-pyrazoline.