Bismuth is a strong oxidizing agent in the pentavalent state. Or pentavalent bismuth is a strong oxidizing agent.

Explanation

Since the inert pair effect is very prominent in Bi, therefore, its +5 oxidation state is much less stable than its +3 oxidation state. In other words, bismuth in the pentavalent state can easily accept two electrons and thus gets reduced to more stable trivalent bismuth.
`Bi^(5+) + 2 e^(-) rarr Bi^(3+)`
Therefore, Bi in the pentavalent state acts as a strong oxidising agent.