  We study the variation of the broad-band spectral energy distribution (SED)
of the BL Lac object Mrk 501 as a function of source activity, from quiescent
to flaring. Through chi-square-minimization we model eight simultaneous SED
datasets with a one-zone Synchrotron-Self-Compton (SSC) model, and examine how
model parameters vary with source activity. The emerging variability pattern of
Mrk 501 is complex, with the Compton component arising from gamma-electron
scatterings that sometimes are (mostly) Thomson and sometimes (mostly) extreme
Klein-Nishina. This can be seen from the variation of the Compton to
synchrotron peak distance according to source state. The underlying electron
spectra are faint/soft in quiescent states and bright/hard in flaring states. A
comparison with Mrk 421 suggests that the typical values of the SSC parameters
are different in the two sources: however, in both jets the energy density is
particle dominated in all states.
