A predetermined volume portion is excited with radio frequency (RF) pulses, the echo signals are detected at different time points after stimulation, and a respective spectral information is determined for each of the echo signals. A peak in the spectral information of the echo signals is detected, and a relaxation time per peak is determined. A substance per peak is determined depending on the relaxation time per peak. An instrumental frequency setting is performed depending on the measured substance resonant frequency. Independent claims are included for the following: (1) a MRI system; (2) a computer program product for frequency calibration of MRI system; and (3) an electronically readable medium storing program for frequency calibration of MRI system.