Precision measurements of the acoustic peaks of the cosmic microwave background indicate that neutrinos must be freely streaming at the photon decoupling epoch when T approximate to 0.3 eV. This requirement implies restrictive limits on secret neutrino interactions, notably on neutrino Yukawa couplings with hypothetical low-mass (pseudo)scalars phi. For diagonal couplings in the neutrino mass basis we find g less than or similar to 1x10(-7), comparable to limits from supernova 1987A. For the off-diagonal couplings and assuming hierarchical neutrino masses we find g less than or similar to 1x10(-11)(0.05 eV/m)(2) where m is the heavier mass of a given neutrino pair connected by g. This stringent limit excludes that the flavor content of high-energy neutrinos from cosmic-ray sources is modified by nu ->nu'+phi decays on their way to Earth.
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