The interactions of pyridine mono-carboxylic acid isomers (PCAIs) (nicotinic acid (NA), isonicotinic acid (INA) and picolinic acid (PA)) with cysteine (RSH) at physiological pH (7.40) have been investigated by square-wave and cyclic voltammetry (SWV and CV), and UV-Vis and infrared spectroscopy. By the addition of isomeric pyridine mono-carboxylic acids, the reduction peak current of mercurous cysteine thiolate could be decreased and also its peak potential E (p) was shifted to more positive values. Also, the significant changes in formal potential E (0)', electron transfer coefficient alpha and electrode reaction standard rate constant k (s) of mercurous cysteine thiolate (Hg-2(SR)(2)) in the presence and absence of PCAIs were observed. The results of voltammetric measurements indicated that binding reactions were occurred between PCAIs and RSH and new electroactive molecular complexes were formed, which resulted in the decrease of free cysteine concentration and the decrease of the reduction peak current of mercurous cysteine thiolate. The logarithmic values of binding constants of NA, INA and PA were calculated as 13.4, 17.7 and 18.9, respectively. The binding ratios for NA-RSH, INA-RSH and PA-RSH complexes were determined as 1: 3, 1: 4 and 1: 4, respectively. Both UV-Vis and FTIR studies also confirmed these interaction reactions.