  Inflationary models involving more than one scalar field naturally produce
isocurvature perturbations. However, while these are fairly well studied, less
is known about their evolution through the reheating epoch, when the
inflationary fields decay into the standard constituents of the present
universe. In this paper, by modelling reheating perturbatively, we calculate
the power spectrum of the non-adiabatic pressure perturbation in three
different inflationary models. We show that the isocurvature can grow large
initially, but decays faster than the pressure perturbations. When reheating
ends, the isocurvature is negligible for the double quadratic and double
quartic inflationary models. For the product exponential potential, which
features large isocurvature at the end of inflation, the isocurvature decays
during reheating and is around five orders of magnitudes smaller than the
pressure perturbation at the end of reheating.
