We investigate the possibility that the Universe is significantly reionized by the decay products of heavy particles .
The ionization produced by decay particles implies a high optical depth even if the maximum level of ionization ever produced is low ( 10 ^ { -2 } ) .
As a consequence , a high ionization fraction ( x \simeq 0.5 ) at high redshifts ( z \simeq 20 ) fails to fit the cosmic microwave background ( CMB ) spectra at l \geq 30 .
Recent CMB data limits the primordial abundance of the decaying particles , favoring long decay times .
Other significant sources of reionization are still needed at z \simeq 13 .
The decay process heats up the medium , bringing the expected y distortion to unobservable levels .