  We examine cosmic microwave background constraints on variations in the
recombination history of the universe. We use a very general extension to the
standard model of recombination characterized by two parameters, a and b, which
multiply the overall rates of recombination and ionization and the binding
energies of hydrogen respectively. We utilize WMAP temperature and TE
cross-correlation data, along with temperature data at smaller scales from
ACBAR and CBI, to place constraints on these parameters. The range of
recombination histories which gives the best fit to the data is -0.6 < log(a) <
0.5 and 0.85 < b < 1.15, forming a diagonal region from low-a/high-b to
high-a/low-b. We find z_rec = 1055 +- 25 and \delta z = 198 +- 5 at 68%
confidence for the range of recombination models which are best able to
reproduce the observed CMB power spectra. Standard recombination provides an
acceptable fit to the data; while there is still room for non-standard
recombination, the allowed variation is small.
