ABSTRACT.Both 3-0x0-(2) and 4-0x0-thiane-1-oxide [,l) exist in the chair form with axial sulfoxide bonds. The axial preference is retained in the dimethyl acetal of 1 b u t not in the dimethyl acetal of k. Beside steric strain, local electrostatic stabilization identified as a gauche 1.4-or a 1,5(g-,g+)-interaction between opposite residual charges rationalize the conformational preferences.