One- and three-component digital recordings from six stations of the Belgian seismic network were used to invert simultaneously for source velocity spectra of 18 aftershocks of the M L 5.8 Roermond earthquake and for site amplification spectra at these stations. The events were selected to have well-constrained fault-plane solutions and the stations to be within 140 km epicentral distance. Frequency-dependent quality factors were computed from the coda waves of the same events. The Joint Source-Site Determination method we used, first corrects the body-wave spectra for radiation pattern, geometrical spreading, attenuation and free surface effect and then is formulated as a linear inverse problem repeated over frequencies (in this application between 1 and 30 Hz), with one reference station for which the site amplification spectrum is given as constant (in this application: La Chartreuse). The unbiased source velocity spectra were fitted with a Brune-type spectral function to obtain a scale factor and a corner frequency, from which, among others, the seismic moment, source radius, stress drop, final disolcation and fracture energy were calculated. In the 1.9–3.4 M L magnitude range, the stress drop, seismic energy and fault radius tend to increase with seismic moment, and seismic efficiency with fault dimension. Brune's stress drop varies between 0.4 and 4.6 MPa. Frictional overshoot prevails over partial stress drop. The average ratio of P - and S -wave corner frequencies is 1.6. The site amplification spectra vary between 1 and 10. Strong site effects were observed at stations Membach and Vianden.