A pressure-equalizing vial access device and method providing closed and sealedreconstitution of vial contents. A rigid container with a fixed internal volumeis connected with a vent lumen extending into the vial. As pressure in the vialincreases, the pressure is equalized with atmospheric pressure by varying thevolume of a compartment within the rigid container. The compartment is formedwith a volume control device that automatically varies the volume of the compartmentin the rigid container to accommodate and equalize the pressure in the vial byincreasing or decreasing the volume of the compartment. In one case the volumecontrol device comprises a sliding disk and in another, a bladder that compresseswith an increase in volume in the container and expands with a decrease.
