  It has been argued that the power spectrum of the anisotropies in the Cosmic
Microwave Background (CMB) may be effectively degenerate, namely that the
observable spectrum does not determine a unique set of cosmological parameters.
We describe the physical origin of this degeneracy and show that at small
angular scales it is broken by gravitational lensing: effectively degenerate
spectra become distinguishable at l ~ 3000 because lensing causes their damping
tails to fall at different rates with increasing l. This effect also helps in
distinguishing nearly degenerate power spectra such as those of mixed dark
matter models. Forthcoming interferometer experiments should provide the means
of measuringotherwise degenerate parameters at the 5-25% level.
