Bismuth is a strong oxidising agent in the pentavalent state.

Explanation

Since the inert pair effect is very prominent in Bi, therefore, its +5 oxidation state in 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 trivalent bismuth.
`Bi^(5+) + 2e^(-) to Bi^(3+)`
Therefore , it acts as a strong oxidising agent .