In this study, we combine CEST-MRI and 31P-MRSI measurements, to better comprehend the underlying mechanisms of the measured changes in APT signal by relating it to the pHi. CEST-MRI and 31P-MRSI were acquired in breast cancer patients receiving neoadjuvant chemotherapy (NAC) treatment before and after the first cycle of NAC at 7T. We found a linear correlation between APT-CEST and pH and a linear correlation between PE/Pi and the pH in the tumor of breast cancer patients before the start of NAC treatment. We hypothesized that an increased pH results in an increased APT signal, yet we found the opposite. As APT mainly depends on mobile amide proton concentration and the exchange rate, this data suggest that the main contributor to the APT signal is the concentration of mobile amide protons.