In the previous publication, some of us reported the conversion of a copper(I) complex to a copper(II) oxalate complex, and claimed that this conversion involved a reduction of CO 2 to oxalate (C 2 O 4 2− ). Herein, we show that the oxalate is produced not by reduction of CO 2 , but by reaction of ascorbate with oxygen. We also present new results that explain in a more comprehensive way the behaviour of these copper compounds under O 2 and CO 2 .Selective reduction of carbon dioxide to C ≥2 compounds using homogeneous metal complexes is a challenging transformation. Only a limited number of examples have been reported over the past decades [1][2][3][4][5][6][7][8][9][10][11][12] . In contrast, there has been a vast increase in reported catalysts for selective CO 2 reduction to C 1 compounds [13][14][15] . Among the examples reported for the reductive coupling of CO 2 to oxalate is a dinuclear Cu complex introduced by some of us in 2014 (ref. 16 ). The in situ generated Cu(I) complex [Cu 2 (m-xpt) 2 ](PF 6 ) 2 (3) formed by reduction of the Cu (II) precursor (1) with sodium ascorbate generated an oxalatebridged dinuclear complex (4), proposed to occur via reductive coupling of atmospheric CO 2 (Fig. 1). Release of the oxalate by addition of mineral acids was described, potentially enabling stepwise conversion of CO 2 into oxalic acid using sodium ascorbate as a comparatively mild reductant. Interestingly, oxidation of ascorbic acid by transition metal compounds, especially those of copper, has been well-known for more than a century 17,18 . Since then, the reaction mechanisms for such oxidations have been intensely studied [18][19][20][21][22] . More specifically, oxidative degradation of ascorbic acid by (a) inorganic oxidants (sodium periodate 23 , sodium hypoiodite 24 ); (b) oxygen 25,26 ; and (c) O 2 in the presence of Gd 27,28 , Co 27 , Pd 29 , Pt 29 , Cd 30 , Fe 31 , or Cu 32 compounds is reported to yield oxalate as a degradation product (see Supplementary Fig. 21 for a typical reaction sequence).We now report that the true origin of the oxalate in the communication published in 2014 is not CO 2 , as it was described, but oxidative degradation of sodium ascorbate.