Abstract: A process for producing isoprene comprising reacting formaldehyde and isobutylene or trimethylcarbinol or with a mixture of isobutylene with trimethylcarbinol in a liquid phase upon heating to a temperature of about 200.degree. C in the presence of a catalyst, viz. sulphamic acid or an aromatic ammonium compound containing a sulpho group in ortho-, meta- or para-position and having the formula: ##STR1## wherein R.sub.1 and R.sub.2 are hydrogen, an alkyl, aryl, alkylaryl, aralkyl;Hydrogen atoms in the benzene ring may be substituted for an alkyl, halide or sulpho group; or in the presence of a catalyst such as an ammonium condensed compound of the naphthalene series containing a sulpho group and having the formula: ##STR2## wherein R.sub.1 and R.sub.2 are hydrogen, an alkyl, aryl, alkylaryl, aralkyl; and as the calalyst use is made of reaction products of said ammonium compounds with formaldehyde, followed by isolation of the desired product.