This paper is devoted to the relationship between properties of the siliceous fl y ash used as an aggregate in production of autoclaved aerated concrete in PGS technology and its properties. Tests were conducted on technological scale. Such technological measurements in this scale are not frequent and the infl uence of mix components, namely fl y ash on the basic functional properties of AAC are seldom found in technical literature. A series of AAC with similar binder content, constant composition and stable properties has been selected for experimental purposes. Fly ash used as an aggregate came from various sources, and the composition and properties were a consequence of both the combustion of coal and co-fi ring with biomass. The results show that, when the amount of binder and its properties are stable, the properties of ACC, especially compressive strength, are dependent on the properties of fl y ash, which fulfi ls the function of aggregate. Linking such fl y ash properties as specifi c surface area, fl y ash water demand and content of unburned carbon may be the basis for a qualitative assessment of raw material in the manufacture of AAC.