The selective, one step oxidation of 2,3,6-trimethylphenol (TMP) to 2,3,6-trimethyl-1,4-benzoquinone (TMQ) by a catalytic amount of CuClz (up to 100 turnovers) and NH,OH . HCl has been investigated. TMQ was obtained in up to 80% yield using only 1.5 wt% CuC12 catalyst (related to the amount of TMP) and 2.8 wt% NH,OH . HCl co-catalyst under ambient conditions. The initial oxidation state of copper had no influence on the rate or selectivity of the reaction. The synergy between catalyst and co-catalyst was most prominent in the low concentration range. Highest TMQ yield was obtained with tert-butyl hydroperoxide (TBHP), whereas the use of molecular oxygen (under pressure) resulted in relatively poor yield. At low TBHP/TMP ratio, the dimer content of the product is significant, but can be efficiently suppressed by applying the oxidant in lo-25 mol% excess to the stoichiometric ratio. The TMQ yield could be improved by adding TMP in portions during oxidation, and by using TBHP in air instead of nitrogen atmosphere. The presence of Cl-ion proved to be important for obtaining good selectivity, but the formation of 4-chloro-2,3,6-trimethylphenol as an intermediate can be excluded, when the amount of catalyst is less than 3 wt%. Electrochemical and catalytic investigations revealed that the reduction of Cu'+ by TMP and its reoxidation by TBHP are fast, but the further oxidation of the dimer intermediate to TMQ is slow.