Investigations of spin dynamics in Ni nanoparticles (NPs) with diametersD = 3.8, 11.7, 15 and 21 nm embedded in an amorphousSiO2 matrix ofcomposition 15/85 (Ni/SiO2) are reported using EMR (electron magnetic resonance) spectroscopy at 9.28 GHz.Three resonance lines are observed whose EMR parameters, namely linewidthΔH, resonancefield Hr andintensity Io, are measured from 5 to 300 K. Line 1 with temperature-independentΔH = 50 Oe andg≈2, and intensityvarying as 1/T, is shown to result from paramagnetic defects in theSiO2 matrix. Lines2 and 3, with g≈2.2 and 8, respectively, and temperature-dependent EMR parameters are assigned to Ni NPs. While line 2with g≈2.2 is due to the majority of Ni NPs, the source of line 3 is discussed in terms oftwo possibilities: (i) large clusters of blocked Ni NPs and (ii) the inherent part ofthe composite asymmetric line made up of lines 2 and 3 predicted by theRaikher–Stepanov (RS) model for dispersed ferromagnets. The temperature dependence ofΔH (full width at half-maximum) of the composite line obtained by integration of the EMRspectra decreases with the increase in temperature, reaching a minimum near 300 K inagreement with the RS model. The observed decreasing asymmetry of the compositeabsorption spectra with increasing temperature is also in agreement with the predictions ofthe RS model, thus providing a satisfactory explanation for the observed temperaturedependence of the EMR spectra of Ni NPs. Large clusters of blocked Ni NPs as the sourceof line 3 are ruled out and additional tests for the RS model are proposed. Thedecreasing magnetization with decreasing particle size of Ni NPs observed here isdiscussed in terms of the possible roles of the surface disordered spins and Ni–SiO2 interaction.