The strength of hydrogen bridge bonds in rows of anthraquinone (AQ) and 9,10-dithioanthracene (DTA) molecules adsorbed at a Cu (111) surface is investigated by means of scanning tunneling microscopy. The temperature-dependent rate of abstraction of molecules from the molecular rows was measured. Evaluation of the results similar to molecular diffusion data indicates an abstraction barrier of 0.13 and 0.16 eV for AQ and DTA, respectively. Given known diffusion barriers, this offers an estimated energy of the (OH)-H-center dot center dot center dot-C and (SH)-H-center dot center dot center dot-C hydrogen bond of 0.05 and 0.02 eV for AQ and DTA, respectively.