The source parameters of 523 aftershocks (0.5 < ML < 5.9) of the 1999 Kocaeli earthquake are determined by performing a two-step spectral fitting procedure. The source spectrum, corrected for both site and propagation effects, is described in terms of a standard ω-square model multiplied by an exponential term of frequency. The latter term is introduced to estimate the high-frequency (f > 12 Hz) fall-off of the acceleration source spectra by computing the κ parameter. The seismic moments obtained range between 1.05 × 1014 and 2.41 × 1017 N m, whereas the Brune stress drops are between 0.002 and 40 MPa. The κ value varies between 0.00 and 0.08 sec, indicating a decay of the acceleration level at the higher frequency part of the spectrum greater than that assumed by the ω−2 model. Both the stress drop and the κ parameter show the tendency of increasing with aftershock magnitude. No evidence of self-similarity breakdown is observed between the source radius and M0. Finally, both the seismic moment and the moment magnitude are compared with the local magnitude to derive new moment–magnitude relationships for the area.