  The reversibility of the transfer of energy from the magnetic field to the
surrounding plasma during magnetic reconnection is examined. Trajectories of
test particles in an analytic model of the fields demonstrate that
irreversibility is associated with separatrix crossings and regions of weaker
magnetic field. Inclusion of a guide field increases the degree of
reversibility. Full kinetic simulations with a particle-in-cell code support
these results and demonstrate that while time-reversed simulations at first
"un-reconnect", they eventually evolve into a reconnecting state.
