The invention relates to α-sulfonylamino-acetonitrile derivatives of the general formula (I) including the optical isomers thereof and mixtures of such isomers, wherein Ar1, and Ar2 independently of each other stand for an optionally substituted aryl or heteroaryl group, R1, and R2 stand independently of each other for hydrogen, optionally substituted C1,-C5alkyl, optionally substituted C2-C5alkenyl, C2-C5alkynyl or optionally substituted C3-C6cycloalkyl; R3 designates hydrogen, C3-C5alkenyl, C3-C5alkynyl or optionally substituted C1,-C5alkyl; R4 is optionally substituted C1,-C5alkyl, optionally substituted C2-C5alkenyl, C2-C5alkynyl or optionally substituted C3-C6cycloalkyl; R5 and R6 are independently of each other hydrogen or optionally substituted C1-C5alkyl, optionally substituted C2-C5alkenyl, C2-C5alkynyl or optionally substituted C3-C6cycloalkyl; R7, and R8 are independently of each other hydrogen or optionally substituted C1,-C5alkyl, optionally substituted C2-C5alkenyl, C2-C5alkynyl or optionally substituted C3-C6cycloalkyl; W designates a bridge selected from —O— —S(O)m- or —NR3—; X designates a direct bond or a bridge selected from —O—, —S(O)m- or —NR3—; a and b independently of each other stand for a number 1, 2 or 3; and c and m independently of each other stand for a number zero, 1 or 2. These compounds possess useful plant protecting properties and may advantageously be employed in agricultural practice for controlling or preventing the infestation of plants by phytopathogenic microorganisms, especially fungi.