N-H-Aziridines undergo efficient ring-opening reactions with aromatic and aliphatic thiols in liquid sulfur dioxide as reaction medium. Due to the Lewis acidic nature of SO2, these reactions do not require any other catalytic additives. The expected β-alkyl/arylthio-amines (β-amino thioethers) are obtained with excellent β-regioselectivity. The developed reaction conditions are compatible with chiral starting materials the enantiomeric purity of which is conserved in the corresponding products. This method is also useful for direct synthesis of carbohydrate–amino acid conjugates and 2-iminothiazolidine derivatives.