6-Thiopurine, 6-thiopurine riboside, 6-thioguanine, 6-thioguanosine, 8-thioguanosine, 2-thiouracil, 4-thiouridine, 2-thiocytosine and 2-thiocytidine are investigated by means of DC polarography and coulometry. In acid medium they give a typical catalytic hydrogen discharge wave, as shown by its parameters and the regeneration of the depolarizer during the electrode process. At pH values above 4.2 ... [Show full abstract] a second catalytic wave appears. Both catalytic waves possess pronounced surface characteristics, most likely due to adsorption of the molecules with differing orientations on the electrode surface. The catalytic wave overlaps the reduction wave, which is placed in evidence under conditions where the catalytic effect is absent. Under the same conditions, 2-thiouracil was found to be catalytically inactive, but it induces catalytic hydrogen evolution in Co(II)-containing ammonia buffer. The present results indicate that the position of thio group yields also such distinct change in the catalytic activity of the investigated compounds.