α-Cyanoacrylates of the formula I, where: R1 is ORa wherein Ra is hydrogen, substituted alkyl, branched alkyl, branched alkenyl, cycloloalkyl, heterocyclyl, aryl, phenylalkyl or alkylimino; is NRbRc wherein Rb is hydrogen, alkyl which may be substituted, alkenyl, alkynyl; Rc is hydrogen, alkyl which may be substituted alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, phenylalkyl or alkylimino; or Rb and Rc form an alkandiyl-chain which may be substituted; or is SRd wherein Rd is hydrogen, alkyl which may be substituted, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, phenylalkyl or alkylimino; R2 is alkyl which may be substituted; is alkenyl or alkynyl; R3 is alkyl may be substituted; is alkenyl or alkynyl; R4 is hydrogen, halogen, cyano or alkyl; and their agriculturally useful salts, processes and intermediates for their preparation; and the use of these compounds or of compositions comprising these compounds for controlling undesirable plants are described.