Effects of the catalytic in vivo chemical environment have often been neglected or underestimated in CEST-MRI studies. Phosphate is the predominant exchange catalyst in intracellular fluid and is essential for biosynthesis and bioenergetics. In this study, we evaluated the influence of phosphate on amine and amide CEST contrast using Bloch-McConnell simulations applied to physical phantom data. We demonstrate that amine proton exchange is greatly catalyzed by phosphate, under a physiological concentration range. We propose that catalytic agents should be considered as confounding factors in future CEST-MRI researches. This new dimension may also benefit the development of novel phosphate-sensitive imaging method.