  We present a modified implementation of the Euclidean action formalism
suitable for studying the thermodynamics of a class of cosmological solutions
containing Killing horizons. To obtain a real metric of definite signature, we
perform a `triple Wick-rotation' by analytically continuing all space-like
directions. The resulting Euclidean geometry is used to calculate the Euclidean
on-shell action, which defines a thermodynamic potential. We show that for the
vacuum de Sitter solution, planar solutions of Einstein-Maxwell theory and a
previously found class of cosmological solutions of N = 2 supergravity, this
thermodynamic potential can be used to define an internal energy which obeys
the first law of thermodynamics. Our approach is complementary to, but
consistent with the isolated horizon formalism. For planar Einstein-Maxwell
solutions, we find dual solutions in Einstein-anti-Maxwell theory where the
sign of the Maxwell term is reversed. These solutions are planar black holes,
rather than cosmological solutions, but give rise, upon a standard
Wick-rotation to the same Euclidean action and thermodynamic relations.
