Cal. Evid. Code § 703.5
No person presiding at any judicial or quasi-judicial proceeding, and no arbitrator or mediator, shall be competent to testify, in any subsequent civil proceeding, as to any statement, conduct, decision, or ruling, occurring at or in conjunction with the prior proceeding, except as to a statement or conduct that could (a) give rise to civil or criminal contempt, (b) constitute a crime, (c) be the subject of investigation by the State Bar or Commission on Judicial Performance, or (d) give rise to disqualification proceedings under paragraph (1) or (6) of subdivision (a) of Section 170.1 of the Code of Civil Procedure. However, this section does not apply to a mediator with regard to any mediation under Chapter 11 (commencing with Section 3160) of Part 2 of Division 8 of the Family Code.
Ca. Evid. Code § 703.5