Reactions occurring within each one of two mixtures, a mixture of deuterated dimethylsulfoxide, DMSOd 6 , with CH 3 I (system I) and another mixture of DMSO-d 6 with CH 3 I, NaOH and water (system II), were monitored by 1D and 2D nuclear magnetic resonance ( 1 H, 13 C, 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 methoxybis(trideuteromethyl)sulfonium iodide, 3; the precipitation of hexadeuterated trimethyloxosulfonium, 2a; a methyl exchange between DMSO-d 6 and 2a to produce trideuterated dimethylsulfoxide, DMSO-d 3 , 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 [Na 4 (DMSO-d x ) 15 ][(I 3 ) 3 I], 1a, a green solid with metallic shine that corresponds to an isotopomer of 1, which is produced by the self-assembly of DMSO and CH 3 I in the presence of NaOH and water.