A representative lignin-type component from biomass burning aerosol has been shown to react with OH radicals in model cloud water yielding colored organic species. In this paper we investigated the chemical properties of the complex reaction products formed from 3,5-dihydroxybenzoic acid. The reaction was followed by UV-VIS spectrophotometry, liquid chromatography, electrospray-mass spectrometry, thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry and a thermal method. This paper provides experimental proofs that actually larger molecular weight species are formed in the aqueous phase by free radical oligomerization. The features observed by all analytical techniques closely resemble those found for natural humic acids and HULIS found in rural and biomass burning aerosol. Therefore such processes are assumed to produce the ubiquitous humic-like substances (HULIS) in atmospheric aerosol. Since these species show intense absorbance in the lower visible to UV range, they might also be important in atmospheric absorption of solar radiation.