A total of 512 male Cobb 500 broiler chickens was used to investigate the additivity of apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of P in mixed diets containing corn and soybean meal (SBM) with or without phytase supplementation. To test the additivity of digestibility values in the mixed diets, the difference between the predicted values calculated from the measured values in the individual ingredients and the determined values in the mixed diets were examined. Birds were fed a starter diet from d 1 to 21 post hatching and then randomly allotted to 7 dietary treatments in a randomized complete block design. Four semi purified diets were prepared to contain corn or SBM as a sole source of P with or without the addition of phytase at 1,000 phytase units/kg diet. Two mixed diets were also prepared to contain corn and SBM with or without the addition of phytase at 1,000 phytase units/kg diet. A P-free diet (PFD) was formulated to estimate the basal ileal endogenous loss (BEL) of P. Chromic oxide was included in all diets as an indigestible marker. Birds were fed the experimental diets for 3 d. On d 24 post hatching, ileal digesta of birds was collected from the distal two-thirds of the ileum. The BEL of P as determined by the PFD was estimated to be 166 mg/kg dry matter intake. The addition of phytase improved (P < 0.05) the AID and SID of P in corn and SBM. The determined AID or SID of P in corn and SBM with or without phytase were used to predict the AID or SID of P in the mixed diets. The predicted AID and SID of P in the mixed diets with or without phytase supplementation were consistent (P > 0.05) with the determined values and were thus additive. The presence of phytase in the mixed diet did not affect the additivity of AID and SID of P. However, the extent of difference between the predicted and determined AID values for the mixed diet containing corn, SBM and phytase was higher than the difference in SID values, while the reverse was the case with the mixed diet containing only corn and SBM.