  Data from solar neutrino and KamLAND experiments have led to a discovery of
nonzero neutrino masses. Here we investigate what these data can tell us about
neutrino interactions with matter, including the poorly constrained
flavor-changing nu_e-nu_tau interactions. We give examples of the interaction
parameters that are excluded by the solar/KamLAND data and are beyond the reach
of other experiments. We also demonstrate that flavor-changing interactions, at
the allowed level, may profoundly modify the conversion probability for
neutrinos of energy <~ 6 MeV and the values of the mass parameter inferred from
the data. The implications for future experiments are discussed.
