Fly ashes originated from the fluidized bed boilers were studied in order to examine their rheological properties.The ashes, used alternatively as aqueous suspension or component of cement pastes, were admixed with two types of superplasticizers (derivates of the acrylic acid and maleic anhydrite).It has been proved that the fly ash suspensions require large water amount (w/s=0.9) and their rheological parameters increase with time.Rheological properties of cement pastes containing the ashes studied significantly differ depending on the superplasticizer applied.This was explained with impact of sulphates present in ash on the superplasticizer effectiveness.