This paper presents the use of spread spectrum technique to reduce electromagnetic interference caused by fast switching transitions of a resonant inverter and rectifier used for wireless power transfer (WPT) systems. We investigate the magnetic field emission from a series-resonant WPT system having the resonance frequency of 20 kHz in a frequency band from 9 kHz to 150 kHz. The averaging spectral estimate (Welch's Periodogram) of the simulated leakage magnetic field shows that a harmonic peak can be reduced up to 8.7 dB by dithering the switching frequency with a triangular modulation profile.