Containing the correspondence, accounts and papers of three Caithness families, the Williamsons of Banniskirk, the Sinclairs of Southdun and the Hendersons of Stemster, Bower. The papers span the seventeenth to twentieth centuries. The bulk of the papers relate to the Ulbster estates in Caithness, under the Lieutenant Colonel Benjamin Williamson's and Captain David Henderson's respective factorships. There are letters relating to The Caithness Fencible Regiment and the Caithness Highlander Regiment, circa 1790-1815.

Letters of special interest include those of Sir John Sinclair of Ulbster (1754-1835), first president of the Board of Agriculture and author of 'A statistical account of Scotland', mainly on Caithness agriculture and politics. There are letters from his son, Sir George Sinclair of Ulbster (1790-1868) and his daughter, the novelist Catherine Sinclair (1800-1864) amongst others.