Abstract We report here a novel reductive coupling reaction of conjugated, non-or poorly enolizable aldehydes induced by H 2 S and operative in aqueous solutions under prebiotically relevant conditions. This reaction leads from retinal to β-carotene, and from benzylic aldehydes to the corresponding diarylethylenes. This novel reaction also opens a new potentially prebiotic pathway leading from glyoxylic acid to various compounds that ...