Chenopodium acerifolium Andrz. and C. karoi (Murr) Aellen have often been confused with each other and with similar taxa occurring in the northernmost parts of Russia. In particular their relationships with C. prostratum Bunge ex Herder nom. illeg. and C. jenissejense Aellen & Iljin have not been clear. The type material of C. jenissejense is heterogeneous and here we designate the lectotype for it so that C. jenissejense will be probably conspecific with C. prostratum, for which the lectotype is also designated here. We provisionally consider C. jenissejense, a Siberian riverside plant, to be conspecific with the mainly Central Asian C. karoi, but further studies are needed to confirm their relationships. Emended morphological descriptions are given of C. acerifolium and C. karoi and their types are discussed. Paul Aellen's role in the taxonomic treatment of C. karoi s. lato and in preparing the account of Chenopodium for the Flora of the USSR is clarified based on the correspondence between Aellen and M.M. Iljin.