Fonds consists of collages (watercolour and fabric) and drawings of costumes designed for the following UVic and community theatre productions: On Baile's Strand, 1965; The Player Queen, 1965; A Man for all seasons, 1965; Renaissance '65, 1965; Murder in the cathedral, 1966; Rhinoceros, 1966; Hadrigan VII, 1973; Old tomorrow: a saga of the CPR, 1973; The three sisters, 1974; The bartered bride, 1978; A Dicken's digest, n.d.; Ghosts, n.d.; Mrs. Alving, n.d.; a midsummer night's dream, n.d. Fonds also includes slides of 3 unidentified productions possibly depicting costumes designed by Biddy Gaddes.

Fonds reflects Siebner’s intellectual and philosophical interests and ideas on art and life; his interactions with galleries, friends, family and patrons; travels; involvement in the Limners; pre-publication work for his monographs and other books; promotion and sale of art; and public art commissions.
The fonds consists of personal writing and poetry, correspondence, sales lists, inventories, exhibition announcements, sketches, clippings, photographs, film and video, prints and drawings.

The fonds consists of material about the genealogy of the Holland Family and some family memorabilia. It includes a book, printed 'for private circulation only' in 1899 (reprinted with additions, 1915) by Thomas Erskine Holland, called "The Hollands of Conway".