  During cosmic recombination, charged particles bind into neutral atoms and
the mean free path of photons rapidly increases, resulting in the familiar
diffusion damping of primordial radiation temperature variations. An additional
effect is a small photon spectrum distortion, because photons arriving from a
particular sky direction were originally in thermal equilibrium at various
spatial locations with different temperatures; the combination of these
different blackbody temperature distributions results in a spectrum with a
Compton $y$-distortion. Using the approximation that photons had zero mean free
path prior to their second-to-last scattering, we derive an expression for the
resulting $y$-distortion, and compute the angular correlation function of the
diffusion $y$-distortion and its cross-correlation with the square of the
photon temperature fluctuation. Detection of the cross-correlation is within
reach of existing arcminute-resolution microwave background experiments such as
the Atacama Cosmology Telescope and the South Pole Telescope.
