We present morphological and molecular evidence of the occurrence of hybrids between great reed warblers Acrocephalus arundinaceus and clamorous reed warblers A. stentoreus in Kazakhstan where both species breed. Three males and one female of 249 examined individuals had wing characteristics that were intermediate between the parental species. Molecular analyses of a nuclear microsatellite locus and a mitochondria) gene confirmed that these four individuals were hybrids: genetic material came from both parental species. Three hybrids carried clamorous reed warbler mitochondria and one carried great reed warbler mitochondria. Our finding does not support hypotheses suggesting that hybridisation occurs as a last resort when conspecific mates are rare, because both species were common in the area. Instead, we suggest that hybridisation might have resulted from mistakes in mate recognition in general and/or that the highly variable song of male great reed warblers might have acted as a supernormal mate choice stimulus for female clamorous reed warblers.