Humic substances (HS) are ubiquitous in the environment, and can act as photosensitizers in the redox reactions of some photochemical processes. The influence of HS in these reactions varies with the HS type and concentration. The total organic carbon content (TOC) of some commercial HS (such as soil and river humic acid, and fulvic acid) was studied. 1-Aminopyrene (1-AP) and 1-hydoxypyrene (1-HP) are carcinogenic and slightly water-soluble polycyclic aromatic hydrocarbons (PAH). The impact of PAH on natural environment is related to their photolysis rates and photoproducts; therefore, it is of interest to study the photolysis of these compounds. Our previous study showed that the photolysis rate of 1-HP was inhibited by HS. In this study, photolysis of 1-AP was conducted with pure water, natural river water, and pure water containing commercial HS. It was found that the photolysis rate of 1-AP can be inhibited or enhanced by HS, depending on the type and concentration. The first order photolysis rate constant of 1-AP (10 M) in phosphate buffer (pH 7.0, 1 mM) containing a humic acid (20-80 ppm) was enhanced by up to 5 folds. With a fulvic acid (20-80 ppm), it was enhanced by about 2 folds. With a soil humic acid, it was enhanced by about 2 folds at the concentration of 20 ppm and was inhibited by up to 4 folds at the concentration of 80 ppm. Atrazine (2-chloro-4-ethylamine-6-isopropylamino-s-triazine) is a widely used herbicide. It is toxic, often bioaccumulative and persistent. In this study, the effect of HS on the photochemical fate of atrazine was also studied. The results showed that photolysis of atrazine can be enhanced by humic acid, depending on the type and concentration of humic acid. The fulvic acid has no effect on its photolysis within 10 days.