We study some properties of the obstacle reactions associated with the
solutions of unilateral obstacle problems with measure data. These results
allow us to prove that, under very weak assumptions on the obstacles, the
solutions do not depend on the components of the negative parts of the data
which are concentrated on sets of capacity zero. The proof is based on a
careful analysis of the behaviour of the potentials of two mutually singular
measures near the points where both potentials tend to infinity.Comment: 18 page