  We obtain spherically-symmetric and $\R^2$-symmetric dyonic black holes that
are asymptotic to anti-de Sitter space-time (AdS), which are solutions in
maximal gauged four-dimensional supergravity, with just one of the U(1) fields
carrying both the electric and magnetic charges $(Q,P)$. We study the
thermodynamics, and find that the usually-expected first law does not hold
unless P=0, Q=0 or P=Q. For general values of the charges, we find that the
first law requires a modification with a new pair of thermodynamic conjugate
variables. We show that they describe the scalar hair that breaks some of the
asymptotic AdS symmetries.
