  Inquisitive modal logic, InqML, in its epistemic incarnation, extends
standard epistemic logic to capture not just the information that agents have,
but also the questions that they are interested in. We use the natural notion
of bisimulation equivalence in the setting of InqML, as introduced in
[Ciardelli/Otto: JSL 2021], to characterise the expressiveness of InqML as the
bisimulation invariant fragment of first-order logic over natural classes of
two-sorted first-order structures that arise as relational encodings of
inquisitive epistemic (S5-like) models. The non-elementary nature of these
classes crucially requires non-classical model-theoretic methods for the
analysis of first-order expressiveness, irrespective of whether we aim for
characterisations in the sense of classical or of finite model theory.
