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We examine the stability and observational consequences of mixing induced by3He burning in the envelopes of first ascent red giants. We demonstrate thatthere are two unstable modes: a rapid, nearly adiabatic mode that we cannotidentify with an underlying physical mechanism, and a slow, nearly radiativemode that can be identified with thermohaline convection. We presentobservational constraints that make the operation of the rapid mode unlikely tooccur in real stars. Thermohaline convection turns out to be fast enough onlyif fluid elements have finger-like structures with a length to diameter ratiol/d > 10. We identify some potentially serious obstacles for thermohalineconvection as the predominant mixing mechanism for giants. We show thatrotation-induced horizontal turbulent diffusion may suppress the 3He-driventhermohaline convection. Another potentially serious problem for it is toexplain observational evidence of enhanced extra mixing. The 3He exhaustion instars approaching the red giant branch (RGB) tip should make the 3He mixinginefficient on the asymptotic giant branch (AGB). In spite of this, there areobservational data indicating the presence of extra mixing in low-mass AGBstars similar to that operating on the RGB. Overmixing may also occur incarbon-enhanced metal-poor stars.

3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits