Liberty Mutual Insurance plans to market for sale 10 St. James Ave. and 75 Arlington St., two office buildings totaling nearly 825,000 square feet that the company owns and partially occupies in Boston’s desirable Back Bay.

10 St James (also known as 319 Stuart St.) is a fully leased, 565,758-square-foot, Class A office building built in 2001. Liberty Mutual occupies 112,000 square feet of the building under a lease that expires next May.

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