The photolysis of 3,3-dimethyl-3H-pyrazoles derived from 2-diazopropane and acetylenic esters and nitriles leads in good yields to electrophilic cyclopropenes Starting from acetylenic ketones, only the 4-keto-3H-pyrazoles give acylcyclopropenes, the photolysis of the 5-keto-derivatives leading mainly to vinylketenes.