Light-absorbing “humic-like” compounds of secondary origin have been consistently reported in partly inorganic aerosols and in fog waters but their formation could not be explained until now. In this work, we demonstrate that amino acid- and ammonium sulfate-catalyzed reactions in water and ionic solutions produce compounds of identical molecular and optical properties and account well for the quantities found in atmospheric particles. For typical aerosol concentrations of amino acids or ammonium sulfate the rate constants of reaction are found to be identical to the one in concentrated sulfuric acid (10–15 M), clearly demonstrating the efficiency of these catalysts. Our results also show that these reactions should be common in aqueous and ionic aerosols, as confirmed by the observations, and significantly impact their absorption index. In particular, previous radiative calculations indicate that they should substantially reduce current estimates of the cooling contribution of sulfate aerosols on climate.