Process for preparation of isoprene

In a process for the preparation of isoprene, which comprises reacting isobutene with formaldehyde in an aqueous medium, in a first stage to form 4,4-dimethyl-m-dioxane, and an aqueous phase containing formaldehyde, and, in a second stage, decomposing the 4,4-dimethyl-m-dioxane to form isoprene and an aqueous phase containing formaldehyde, at least one of said aqueous phases containing high boiling impurities and at least one of said aqueous phases containing impurites which are readily volatile constituents, the improvement which comprises combining the aqueous phases containing formaldehyde formed in said first and second stages, distilling the resulting material to separate the higher-boiling constituents as a sump product and aqueous formaldehyde containing the readily volatile impurities as distillate, extracting the distillate with a stream of C.sub.4 -hydrocarbons, containing isobutene to remove the readily volatile constituents, feeding the stream of C.sub.4 -hydrocarbons leaving the extraction into the first reaction stage, and mixing the extracted aqueous formaldehyde solution with an alkaline material or conducting it over a basic ion exchanger, at some point after said distillation, rectifying the resulting solution to produce substantially all the formaldehyde as head product, and an aqueous effluent substantially free from formaldehyde as sump product.