  Small Recent theories have proposed a variety of massive particles, like the
moduli, whose abundance or decay endangers standard cosmological results. To
dilute them, thermal inflation has been proposed, with its own massive scalar
flaton field which goes on decaying in the era of Mev-scale temperatures. In
this paper, the effect of late-time entropy production on neutrino decoupling
during such reheating is investigated by including a term, arising from the
rate of entropy production due to scalar decay, in the Boltzmann equation for
the neutrino number density. The effect on the decoupling temperature of
massless neutrinos is studied. It is found that a lower bound to the scalar
decay rate constant can be set at 10^{-22} Gev.
