The archives consist of documents which were created by the prefectures, deaneries and parishes; personal materials of some priests who worked in the diocese; and documents of the Chapters of Koper.
The most precious records are the parchment documents. The Archives contain transcripts of parish records dating back to 1835, however the pre-1949 records of the Koper Diocese itself, which was united with Trieste Diocese in 1828, have not yet been returned from Trieste, and the parish archives are still kept in the parishes.
The publications Guide to the Archives of Slovenia [Vodnik po arhivih Slovenije] (1965, pp 555–565, 94) and Guide to Registers in the Territory of the SR Slovenia [Vodnik po matičnih knjigah za območje SR Slovenije] (1972–1974, three volumes) list all church archives.
The collections could be accessed in situ. A photocopy or a digital copy on CD-ROM of most recent documents (the last 70 years) are available on request.