Heteroaryloxy-acetamides of the general formula (I) ##STR1## in which Het represents thiadiazolyl which is substituted by halogen or by C.sub.1 -C.sub.4 -alkyl or phenyl, each of which is optionally substituted by halogen, andAr represents optionally halogen-, C.sub.1 -C.sub.4 -alkyl- or C.sub.1 -C.sub.4 -halogenoalkyl-substituted phenyl.can be used with good results as selective herbicides in rice cultivation.The novel herbicidal active compound combinations comprising (1) a heteroaryloxy-acetamide of the formula (I) above and (2) in each case one active compound from the group of already known selective rice herbicides (e.g. bensulfuron, MCPB, and thiobencarb) exhibit, at certain weight ratios, synergistic activities and can be used particularly advantageously as selective herbicides in rice crops.