Mass. Gen. Laws ch. 233 § 23C
All memoranda, and other work product prepared by a mediator and a mediator's case files shall be confidential and not subject to disclosure in any judicial or administrative proceeding involving any of the parties to any mediation to which such materials apply. Any communication made in the course of and relating to the subject matter of any mediation and which is made in the presence of such mediator by any participant, mediator or other person shall be a confidential communication and not subject to disclosure in any judicial or administrative proceeding; provided, however, that the provisions of this section shall not apply to the mediation of labor disputes.
For the purposes of this section a "mediator" shall mean a person not a party to a dispute who enters into a written agreement with the parties to assist them in resolving their disputes and has completed at least thirty hours of training in mediation and who either has four years of professional experience as a mediator or is accountable to a dispute resolution organization which has been in existence for at least three years or one who has been appointed to mediate by a judicial or governmental body.
Mass. Gen. Laws ch. 233, § 23C