In this work, electron magnetic resonance (EMR) spectroscopy and magnetometry studies were employed to investigate the origin of the observed room-temperature ferromagnetism in chemically synthesized Sn1-xFexO2 powders. EMR data clearly established the presence of two different types of signals due to the incorporated Fe ions: paramagnetic spectra due to isolated Fe3+ ions and broad ferromagnetic resonance (FMR) spectra due to magnetically coupled Fe3+ dopantions. EMR data analysis and simulation suggested the presence of high-spin (S = 5/2) Fe3+ ions incorporated into the SnO2 host lattice both at substitutional and at interstitial sites. The FMR signal intensity and the saturation magnetization Ms of the ferromagnetic component increased with increasing Fe concentration. For Sn0.953Fe0.047O2 samples, well-defined EMR spectra revealing FMRs were observed only for samples prepared in the 350–600°C range, whereas for samples prepared at higher annealing temperatures up to 900°C, the FMRs and saturation magnetization were vanished due to diffusion and eventual expulsion of the Fe ions from the nanoparticles, in agreement with data obtained from Raman and X-ray photoelectron spectroscopy.