  In this paper we explore the decay $\eta_b --> \tau^+ \tau^-$ as a probe for
a light pseudoscalar or a light axial vector state. We estimate the standard
model branching ratio for this decay to be $~4\times 10^{-9}$. We show that
considerably larger branching ratios, up to the present experimental limit of
~8%, is possible in models with a light pseudoscalar or a light axial vector
state. As we do not include possible mixing effects between the light
pseudoscalar and the $\eta_b$, our results should be reliable when the
pseudoscalar mass is away from the $\eta_b$ mass.
