DMSO and acetone have almost identical structures, except for us replacing the carbonyl carbon with sulfur to obtain DMSO. DMSO's melting point is 19 oC, whereas acetone's is -95 oC.

Is the dominant factor DMSO's larger molecular weight? Probably not. While acetone is 58 g/mol vs. DMSO's 78 g/mol, 2-pentanone is 86 g/mol, but still has a very low melting temperature of -77 oC.

The only protons on all of these species are aliphatic, so hydrogen bonding is probably not the dominant contributor to this effect either.

What's going on?!