The structure and solid-state luminescence properties of an EuIII com­pound with two different lanthanide sites, [Eu(μ-O)5(OH)(H2O)2][Eu(DOTA)(H2O)]2 (DOTA is 1,4,7,10-tetra­zacyclo­dodecane-1,4,7,10-tetra­acetate, C16H24N4O8), were determined. The com­pound crystallizes in a laminar structure in the triclinic space group P\overline{1}, where the two sites are a free europium(III) ion and an [Eu(DOTA)(H2O)]− com­plex. The crystal structure was determined using com­plex data treatment due to nonmerohedral twinning. Experimental data sets were recorded with large redundancy and separated according to scattering domains in order to obtain a reliable structure. In the first site, the [Eu(DOTA)(H2O)]− com­plex was found to adopt a capped twisted square-anti­prismatic (cTSAP) conformation, where a capping water mol­ecule increased the coordination number of the europium(III) site to nine (CN = 9). In the second site, the europium(III) ion was found to be coordinated by two water mol­ecules, one hydroxide group and five oxide groups from neighbouring [Eu(DOTA)(H2O)]− com­plexes. The coordination geometry of this site was found to be a com­pressed square anti­prism (SAP) and the coordination number of the europium(III) ion was found to be eight (CN = 8). A large increase in the rate constant of luminescence was observed for EuIII in [Eu(DOTA)(H2O)]− in solid-state luminescence spectroscopy measurements com­pared to in solution, which led to investigations of single crystals in deuterated media to exclude additional effects of quenching. We conclude that the most probable cause of the decrease in the observed luminescence lifetimes is the high asymmetry of the coordination environment of [Eu(DOTA)(D2O)]− in the [Eu(μ-O)5(OD)(D2O)2][Eu(DOTA)(D2O)]2 crystals.