EPR spectroscopy is extremely sensitive to the presence of S = 1/2 copper centers, which is evident from the fact that structural defects in the MOP and MOF samples could be clearly 10 visualized. While MCu PWs (mixing level 1) detection is thus possible with high sensitivity, the determination as to whether a framework with mixing level 2 or 3 is formed with complete fidelity relies whether the correct IS is obtained. We thus set out to prepare benchmark samples, so that the structomer-selectivity of a synthesis could be determined by comparison with a highfidelity reference. In order to directly visualize how the Rh centers are distributed over kofo-2 samples, we set out to prepare mixed-metal MOPs with appreciable solubility which could be analyzed by mass spectrometry. High-resolution mass spectrometry pairs excellent sensitivity with an ability to determine the sum formula of individual M24 cages, so that the number of Rh atoms present within individual cages could be directly determined. The preparation of soluble mixed-metal MOP cages required the use of L1 = H2BDC-OH, which, unlike RhCu-kofo-2 (L1 = 20 BDC-t Bu), could not be prepared by direct synthesis (Fig. ). However, cation exchange of Cu24(BDC-OH)24 with RhCl3 proceeded readily in a range of alcoholic solvents to deliver RhCu MOPs, the Rh content of which could be tuned by the choice of reaction time and solvent (Fig. , b and Supplementary Table ). Subsequent analysis by electrospray ionization mass spectrometry (ESI-MS, Supplementary Figs. ) furnished signals of intact tetraanionic and pentaanionic mixed-metal cages, which could be assigned to M24 cages containing varying numbers of Rh atoms. The experimentally determined distribution of RhxCu24-x MOP cages for samples with different Rh:Cu ratios verified that Rh centers were unfailingly installed in sets of two, and isolated Rh centers were not present even if the Rh content was kept low (Fig. ). A bombardment model that accounts for Rh depletion in solution permitted us to extract that the rate of Rh entering a RhCu paddlewheel is >1000 times larger than the rate of Rh entering a Cu2 paddlewheel (Supplementary Figs. ). Mass spectrometric analysis thus confirmed that level 2 mixing is obtained with complete fidelity over a wide range of Rh:Cu concentrations. Isolated Cu24 cages were present for RhCu-kofo-2 with low Rh content but disappear for Rh:Cu = 10 35:65, while cages containing an uneven number of Rh centers were never detected (Supplementary Fig. ). The experimental distributions were surprisingly sharp, so that in the case of Rh0.48Cu0.52-kofo-2, no cages containing fewer than 8 or more than 16 Rh atoms could be detected, which deviates notably from a simple exchange model (Fig. and Supplementary Fig. ). RhCu-kofo-2 was thus formed with exclusive level 2 mixing, and any adventitious contamination with level 1 or level 3 mixing remained below the detection limit of mass spectrometry. EPR analysis of Rh0.50Cu0.50-kofo-2 (L1 = BDC-OH), for which extreme fidelity of the targeted mixing level had been ascertained, permitted us to verify that perfect level 2 mixing corresponds to IS = 5(2) for compounds with a 1:1 ratio of Cu and a heterometal. Excellent agreement with the IS values obtained for kofo-2 samples containing different ligands (L1 = 20 BDC-t Bu, IS = 5(2)) or heterometals (Mo instead of Rh, IS = 4(2)) confirmed that a single structomer could be accessed with high fidelity for different metal combinations and L1 ligands (Supplementary Fig. ). The fidelity with which level 3 mixing was achieved in RhCu-kofo-3 (L1 = BDC-OH) was likewise probed by mass spectrometry: no cages containing both Rh and Cu could be detected so that adventitious incorporation of cages with mixing level 2 could be excluded within the detection limits of mass spectrometry.