C₃₃H₃₆0₆.,1/2C₃H₈OS, trigonal, P3₁21, α = 13.538 (3), c = 30.598 (16)Å, Z = 6, D_m = 1.177, D_x = 1.179 Mg m^-3, μ = 0.1154 mm^-1. Data were collected at room temperature. Final R = 0.049 for 935 observed reflections. The present structure analysis is mainly concerned with the determination, by crystallographic means only, of the enantiomeric ratio of the caged ethyl methyl sulfoxide (EMSO) molecules in a single crystal of the tri-o-thymotide (TOT) clathrate.