Desorption electrospray ionization (DESI) has been undergoing rapid developments since its first introduction in 2004.[1] Its current progress and future direction along with those of other ambient ionization methods [2] are highlighted in a recent review.[3] Of particular relevance to this work is the so-called reactive DESI experiment [4] in which one can dramatically improve the performance of DESI by selectively adding a reagent into the spray solvent. Because of the specific reaction between the reagent additive and the analyte, reactive DESI has been demonstrated to be a rapid and sensitive method for detecting a variety of compounds and for reactive imaging in complex matrices.[3, 5] Three important aspects of reactive DESI should be clarified here. First, the process is heterogeneous [6, 7] in the sense that the reagent in the spray solvent is precharged and the analyte on the surface is neutral before they mix together on the sampling surface. Second, the method is limited to covalent products with only a few exceptions of noncovalent complexes such as crown ether-oseltamivir [8] and alkali metal ion-explosive.[9] Third, for all the aforementioned reactive DESI practices, the reagents and analytes are structurally different. Until now, there is no report of the scenario in which the reagent and analyte are homologues, especially when the homologues can form noncovalent magic number clusters. We consider the quintets [10] of uracil and its homologues as ideal probes for gaining additional insights into the process of reactive DESI and as the proof-of-principle cases in fast screening of candidates with similar properties using reactive DESI. In fact, we have first observed thymine quintet as a magic number cluster by DESI-MS.[11] We have further proposed its structural model [10] and investigated its higher order clusters [12] by ESI-MS and density functional theory (DFT) calculations. From the structural model,[10] one can expect that homologues of uracil with substitute group (s) at its C-5 and/or C-6 sites may also form quintet clusters. If there are two such homologues in the system, they may compete to cluster into the same quintet and a series of hetero-subunit quintets will be produced. These quintets can be readily observed by mass spectrometry as in the ESI-MS study of various coordination complexes because of ligand exchange reactions.[13] Although ESI-MS may be suitable for measuring ligand exchange reactions that occur in seconds or a longer timescale, reactive-DESI-MS can access reaction intermediates with lifetimes on the order of milliseconds.[14] Thus, we believe that new insights may also be obtained for the formation of these magic number clusters under the very fast process of reactive DESI. Here, we use reactive DESI to investigate the formation of quintets of uracil homologues by placing two different homologues separately in the spray solvent and on the surface. Exchange reactions for the subunit (s) of the quintet are observed. Furthermore, the different signal distributions of the homo-subunit and heterosubunit quintets formed from any two of the uracil homologues can be compared by using the values of a specially defined molar ratio Rm. The results provide a relative stability order for the quintets formed by each of the homologues, which is in agreement with that obtained by DFT calculations. The series of homologues including uracil (U), thymine (T), 6-methyluracil (6MU), 5-ethyluracil (5EU), and 5, 6-dimethyluracil (DiU) were dissolved in methanol to make stock solutions at the concentration of 0.01 M. They were diluted to the specified concentrations by methanol before being spread over a polymethylmethacrylate (PMMA …