Unveiling dissolved species in electrodeposition baths helps our understanding of electrodeposition behavior, such as growth orientation. A highly concentrated aqueous alkaline copper(II)-lactate solution is used for the electrodeposition of copper(I) oxide (Cu2O) thin films with <111> orientations; the semiconductor properties of these films facilitate their use in solar-cell materials, photocathodes, and photocatalysts. However, the dissolved species, presumably copper(II)-lactate complexes, cannot be deduced on the basis of known thermodynamic data, and have not been convincingly determined yet. In this work, we determine these cupric complexes by pH titration, ultraviolet-visible spectroscopy, and electrospray- ionization mass spectrometry (ESI-MS), including probe-ESI-MS (PESI-MS). Using PESI-MS, we successfully analyzed a highly concentrated solution without sample dilution. The determined complexes are Cu(H-1L)L- and Cu(H-1L)(2)(2-), where the H-1L2- (CH3CH(O-)COO-) is a lactate ion with a deprotonated a-hydroxyl group. As far as we know, this is the first direct experimental observation of H-1L2- ions in a highly concentrated aqueous alkaline copper(II)-lactate solution. We also propose that H-1L2- is stabilized by the high concentration and through coordination to copper(II) ions. (C) The Author(s) 2018. Published by ECS.