Reactions occurring within each one of two mixtures, a mixture of deuterated dimethylsulfoxide, DMSO-d6, with CH3I (system I) and another mixture of DMSO-d6 with CH3I, NaOH and water (system II), were monitored by 1D and 2D nuclear magnetic resonance (1H, 13C, heteronuclear multiple quantum correlation, heteronuclear multiple bond correlation and diffusion-ordered NMR spectroscopy). The analysis of the spectra as a function of reaction time revealed the formation of methoxy-bis(trideuteromethyl)sulfonium iodide, 3; the precipitation of hexadeuterated trimethyloxosulfonium, 2a; a methyl exchange between DMSO-d6 and 2a to produce trideuterated dimethylsulfoxide, DMSO-d3, 4, and nona-deuterated trimethyloxosulfonium iodide, 2b; and the production of small quantities of methanol, 5, trideuterated dimethylsulfide, 6, and dimethyl ether, 7, in both systems. Only system II precipitated deuterated [Na4(DMSO-dx)15][(I3)3I], 1a, a green solid with metallic shine that corresponds to an isotopomer of 1, which is produced by the self-assembly of DMSO and CH3I in the presence of NaOH and water.