Photo courtesy of Newmark

Blackstone’s office real estate arm is testing the investment sales climate in Boston with plans to sell a Back Bay multitenant building after an 11-year ownership period.

EQ Office is listing the 42-year-old boutique office building at 399 Boylston St. through brokerage Newmark, according to a pair of real estate industry sources.

Completed in 1983, the 13-story building totals 233,376 square feet and previously traded for $117 million in 2014, or over $500 per square foot.

Currently the building is 90 percent leased, according to data by brokerage Hunneman, compared with 87 percent at the time of EQ Office’s acquisition.

Although the building’s largest tenant, Natixis Global Asset Management, relocated to 888 Boylston St. in 2017, owners successfully attracted new tenants, including the relocation of business and technology consultants Slalom’s local offices in 2018. In 2024, EQ Office announced completion of four leases totaling over 21,000 square feet to tenants including ProKidney, Rip Road Capital Partners and Valspring Capital.

A spokeswoman for EQ Office declined to comment on the potential sale.

Following a series of deeply discounted sales in the past year, some office buildings with strong occupancy have traded for higher prices than their previous sale.

Last month, Cambridge-based North Colony Asset Management paid $48 million for 343 Congress St. in Fort Point, an 18 percent premium on its previous trade in 2015. Office buildings at 3 Speen St. in Framingham and 199 Wells Ave. in Newton also have traded this year above their previous sales prices, according Hunneman’s office market report.

Back Bay’s vacancy rate remains the lowest of Boston’s major office submarkets, according to a CBRE report, at 15.8 percent. The Back Bay had over 61,000 square feet of positive absorption during the first quarter, and average asking rents of $66.02 per square foot.