A series of batch experiments were conducted to determine the adsorption and desorption characteristics of divalent mercury (Hg(II)) by six humic substances (HS) (three humic acids (HA) and three fulvic acids) extracted from three dark brown forest soils from Mount Tai, Laoshan Mountain, and Fanggan in Shandong Province, China. It was found that Hg(II) adsorption isotherms could be well fitted with both Langmuir and Freundlich equations. The three HA had larger adsorption intensity and adsorptive capacity than those of the three fulvic acids. The desorbed percentages of the three HA were less than 2.6%, which suggests that HA had a high binding strength for Hg(II). As the HA showed stronger adsorptive intensity and lower desorption ratio for Hg(II), the HA might play an important role in the adsorption/desorption processes in soil for Hg(II). The Fourier transform infrared (FTIR) spectroscopy showed that the six HS contained the same function groups and that the adsorption of Hg(II) on HS was mostly a result of ion exchange and the formation of hydrogen bonds, which may take place on the carbonyls (C=O), carboxyls (COO-), hydroxyls (O-H), and C-O of the HS.