  We present a gauge-invariant formalism to study the evolution of curvature
perturbations in a Friedmann-Robertson-Walker universe filled by multiple
interacting fluids. We resolve arbitrary perturbations into adiabatic and
entropy components and derive their coupled evolution equations. We demonstrate
that perturbations obeying a generalised adiabatic condition remain adiabatic
in the large-scale limit, even when one includes energy transfer between
fluids. As a specific application we study the recently proposed curvaton
model, in which the curvaton decays into radiation. We use the coupled
evolution equations to show how an initial isocurvature perturbation in the
curvaton gives rise to an adiabatic curvature perturbation after the curvaton
decays.
