Cyanomethyl 1-thioglycosides ofD-galactose, D-glucose, 2-acetamido-2-deoxy-D-glucose, and D-mannose were prepared from the respective pseudothiourea derivatives and chloroacetonitrile. The nitrile group in these cyanomethyl thioglycosides can be converted to a methyl imidate group by treatment with sodium methoxide or HC1 in dry methanol to yield 2-imino-2-methoxyethyl 1-thioglycosides (IME-thioglycosides). The factors influencing the yield of IME-thioglycosides were investigated. The most convenient method of preparing IME-thioglycosides was treating 0.1 M cyanomethyl thioglycoside peracetate in dry methanol with 0.01 M sodium methoxide at room temperature for 24-48 h (50-60% yield). These IME-thioglycosides reacted readily with simple amines, amino acids, and proteins in mildly alkaline buffer solutions. Alpha-amylase and lysozyme modified with these reagents under appropriate conditions retained full activities. Thus the IME-thioglycosides constitute a new group of reagents for attaching sugars to proteins.