Mass. Gen. Laws ch. 59 § 52B
All information collected pursuant to section 8A of chapter 58A and sections 38D, 38E, 57D and 61A of this chapter shall be open to the inspection of the assessors, the commissioner, the deputies, clerks and assistants of either the assessors or the commissioner and such other officials of the commonwealth or of its political subdivisions who have occasion to inspect such information in the performance of their official duties, but to no other person except by order of the appellate tax board or a court, except that if the assessor bases a valuation of an assessed owner's real or personal property, in whole or in part, on a comparable sale, or sales, the assessor shall provide any market data relating to such comparable sale or sales to the assessed owner of the property or his designated representative upon request. Nothing in this section shall prevent a person who submitted that information, or his designated representative, from inspecting or being provided a copy of the submission upon request.
Mass. Gen. Laws ch. 59, § 52B