220 ILCS 5/10-106
The fees of witnesses for attendance and travel shall be the same as fees of witnesses before the circuit courts of this State, such fees to be paid when the witness is excused from further attendance, when the witness is subpoenaed at the instance of the Commission, or any commissioner or administrative law judge ; and the disbursements made in the payment of such fees shall be audited and paid in the same manner as are other expenses of the Commission. Whenever a subpoena is issued at the instance of a complainant, respondent, or other party to any proceeding before the Commission, the Commission may require that the cost of service thereof and the fee of the witness shall be borne by the party at whose instance the witness is summoned, and the Commission shall have power, in its discretion, to require a deposit to cover the cost of such service and witness fees and the payment of the legal witness fee and mileage to the witness when served with subpoena. A subpoena issued as aforesaid shall be served in the same manner as a subpoena issued out of a court.
Any person who shall be served with a subpoena to appear and testify, or to produce books, papers, accounts or documents, issued by the Commission or by any commissioner or administrative law judge , in the course of an inquiry, investigation or hearing conducted under any of the provisions of this Act, and who refuse or neglect to appear, or to testify, or to produce books, papers, accounts and documents relevant to said inquiry, investigation or hearing as commanded in such subpoena, shall be guilty of a Class A misdemeanor.
Any circuit court of this State, upon application of the Commission, or a commissioner or administrative law judge , may, in its discretion, compel the attendance of witnesses, the production of books, papers, accounts and documents, and the giving of testimony before the Commission, or before any such commissioner or administrative law judge , by an attachment for contempt or otherwise, in the same manner as production of evidence may be compelled before the court.
The Commission or a commissioner or administrative law judge or any party may in any investigation or hearing before the Commission, cause the deposition of witnesses residing within or without the State to be taken in the manner prescribed by law for like depositions in civil actions in the courts of this State and to that end may compel the attendance of witnesses and the production of papers, books, accounts and documents.
The Commission may require, by order served on any public utility in the manner provided herein for the service of orders, the production within this State at such time and place as it may designate, of any books, accounts, papers or documents kept by any public utility operating within this State in any office or place without this State, or, at its option, verified copies in lieu thereof, so that an examination thereof may be made by the Commission or under its direction.
220 ILCS 5/10-106