6,7-Dimethyl-5,6,7,8-tetrahydropterin (6,7-DMTHP) is an effective and widely used pseudo cofactor for phenylalanine hydroxylase in the hydroxylation reaction of phenylalanine. 6,7-DMTHP and other tetrahydropterins methylated at C(6) and C(7) such as 2-dimethylamino-3,6,7-trimethyl-5,6,7,8-tetrahydropterin (2-DMTHP), 3,6,7,8-tetramethyl-5,6,7,8-tetrahydropterin (3,6,7,8-TMTHP) and 2-dimethylamino-6,7,8-trimethyl-5,6,7,8-tetrahydropterin (2-DMTTHP) give rise to three oxidation peaks at the pyrolytic graphite electrode. Peak I(, a) is an almost reversible 2e-2H('+) oxidation reaction to an unstable quinonoid-dihydropterin. Based on electrochemical, spectroelectrochemical and kinetic results it is believed that this intermediate has an ortho structure. Cyclic voltammetry thin-layer spectroelectrochemistry and potentiostatic experiments reveal that the quinonoid-dihydropterin undergoes a pH-dependent, first-order reaction to give the corresponding 7,8-dihydropterin. Peak II(, a) is due to a 2e-2H('+) quasi-reversible electrooxidation of covalently hydrated 7,8-dihydro derivative giving rise to two unstable quinonoid-dihydropterins which in turn react chemically to give the corresponding pterin. Nonhydrated 7,8-dihydro derivatives are oxidized to the pterin derivative in the peak III(, a) process.