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, 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 electroms and this gets reduced to more stable trivalent bismuth
`Bi^(5)+2e_(-)to Bi^(3+)`
Therefore, Bi in the pentavalent state acts as a strong oxidixing agent.