  We test the assumption of entropy conservation between Big Bang
nucleosynthesis and recombination by considering a massive particle that decays
into a mixture of photons and other relativistic species. We employ Planck
temperature and polarization anisotropies, COBE/FIRAS spectral distortion
bounds, and the observed primordial deuterium abundance to constrain these
decay scenarios. If between $56\%$ and $71\%$ of the decaying particle's energy
is transferred to photons, then $N_{\mathrm{eff}}$ at recombination is
minimally altered, and Planck data alone allows for significant entropy
injection. If photons are injected by the decay, the addition of spectral
distortion bounds restricts the decay rate of the particle to be $\Gamma_Y >
1.91\times10^{-6} \text{s}^{-1}$ at $95\%$ confidence level (C.L.). We find
that constraints on the energy density of the decaying particle are
significantly enhanced by the inclusion of bounds on the primordial deuterium
abundance, allowing the particle to contribute at most $2.35\%$ ($95\%$ C.L.)
of the energy density of the universe before decaying.
