The aim of this study was to determine whether the serum concentration of the phosphate (Pi) and the Ca x P value correlate with the IRIS stage of chronic kidney disease (CKD) in cats and, thus, whether they can be used as markers of the disease progression. Another aim was to assess whether the concentration of Ca in blood needs to be corrected based on the albumin concentration. The study was performed on 165 cats divided into five groups: the healthy group – C and study groups: I, II, III and IV with cats assigned to the groups based on the IRIS scale. Blood was collected from all the animals. The product of Ca x Pi, Cacorr and the product of Cacorrx Pi were calculated based on the obtained results. Despite no differences between groups I-III, there was a clear upward trend in the Pi concentration, in the Ca x Pi and in the Cacorr x Pi with CKD progression. In group IV, the Pi concentration and the Ca x Pi as well as the Cacorr x Pi value were significantly higher than the other groups. The concentration of Ca and its albumin-corrected serum values did not differ significantly. The serum concentration of Pi and the Ca x P product cannot be used as indicators of CKD progression in cats, but they may be used as additional elements in the diagnosis of stage IV CKD. The results also suggest that the serum calcium concentrations do not need to be albumin-corrected in cats.