Second generation superplasticizers based on polycarboxylates were investigated in terms of their infl uence on the properties of autoclaved aerated concrete (ACC). The macromolecules of the plasticizers contained polycarboxy and polyether segments, and free carboxylic groups gaining solubility of the plasticizer in water. Water solutions of the plasticizers of concentration of 30 wt% were applied as admixtures to concrete. Concrete samples were based on gypsum, calcium oxide and fl y ash as binder and an aggregate. The concrete was produced by foam-gas-silicate (FGS) technique. Fresh mixtures both containing and not containing superplasticizers were autoclaved, and the hardened samples were characterized. The samples containing superplasticizer in amounts enabling to reduce water quantity to 20% showed density about 10% higher, and compressive strength about 20% higher comparing to the samples without plasticizers. Phase analysis did not show any differences between the concrete samples containing or not containing the admixtures. All samples contained tobermorite, hydrogranates, and C-S-H phase.