[en] To determine residence time distribution (RTD) in the phosphate treatment reactor we apply radiotracer method. However this method becomes effective and profitable only if the radiotracer is the suitable one. The problem to resolve in our study is “which phase of the phosphate slurry our injected radiotracer is following?”. For that we add radiotracer to the phosphate slurry and we proceed to phase separation using a filtration system. We follow simultaneously -using appropriate detectors- the behavior and the quantity of the radiotracer in both phases during the separation stage. The same experiment is applied twice using successively "9"9"mTc and "1"3"1I radiotracers. The comparative study proves that "1"3"1I is more suitable than "9"9"mTc for solid phase labeling of phosphate slurry.