Massachusetts General Laws Part II. Real and Personal Property and Domestic Relations (Ch. 183-210) Ch. 186, § 15C

No lease relating to residential real estate shall contain a provision which obligates a lessee to make payments to the lessor on account of an increased real estate tax levied during the term of the lease, unless such provision expressly sets forth (1) that the lessee shall be obligated to pay only that proportion of such increased tax as the unit leased by him bears to the whole of the real estate so taxed, (2) the exact percentage of any such increase which the lessee shall pay, and (3) that if the lessor obtains an abatement of the real estate tax levied on the whole of the real estate of which the unit leased by the lessee is a part, a proportionate share of such abatement, less reasonable attorney's fees, if any, shall be refunded to said lessee. Any provision of a lease in violation of the provisions of this section shall be deemed to be against public policy and void.

If the exact percentage of any such increased tax contained in such a provision is found to exceed that proportion of such increased tax as the lessee's unit bears to the whole of the real estate so taxed, then the lessor shall return to the lessee that amount of the tax payment collected from the lessee which exceeded the lessee's proportionate share of the increased tax, plus interest calculated at the rate of five per cent per year from the date of collection.