A pressure-equalizing vial access device and method providing closed and sealed reconstitution of vial contents. A rigid container with a fixed internal volume is connected with a vent lumen extending into the vial. As pressure in the vial increases, the pressure is equalized with atmospheric pressure by varying the volume of a compartment within the rigid container. The compartment is formed with a volume control device that automatically varies the volume of the compartment in the rigid container to accommodate and equalize the pressure in the vial by increasing or decreasing the volume of the compartment. In one case the volume control device comprises a sliding disk and in another, a bladder that compresses with an increase in volume in the container and expands with a decrease.