A series of isothermal investigations have been performed in static autoclaves at 623 K in both standard start of cycle coolant and a series of more concentrated lithium and lithium-boron solutions to simulate the environment which might result within a porous oxide due to hideout. In solutions containing 220 ppm of lithium, the presence of as little as 50 ppm of boron had a marked ameliorating effect on the corrosion rates of Zircaloy-4. The quantity of lithium which became incorporated in the scale was less in the presence of boron and, in general, decreased as the boron concentration in the bulk solution increased. Preliminary results under heat flux in similarly concentrated environments have suggested that the results are also applicable to the real situation in a reactor, since the volatilization of boric acid under boiling was not so great that boron was prevented from reducing the corrosion rate. In contrast, experiments with the same lithium concentration but just 10 ppm of boron resulted in consistently higher weight gains than in the lithium-only test, suggesting that the concentration of boron must exceed a certain critical value before corrosion can be inhibited, and that concentrations below this threshold increase corrosion rates.