  We present a consistent implementation of weak decays involving an axion or
axion-like particle in the context of an effective chiral Lagrangian. We argue
that previous treatments of such processes have used an incorrect
representation of the flavor-changing quark currents in the chiral theory. As
an application, we derive model-independent results for the decays $K^-\to\pi^-
a$ and $\pi^-\to e^-\bar\nu_e a$ at leading order in the chiral expansion and
for arbitrary axion couplings and mass. In particular, we find that the
$K^-\to\pi^- a$ branching ratio is almost 40 times larger than previously
estimated.
