Nicholas I, Pope
- Biography of Pope St. Nicholas I, also called Nicholas the Great.
- http://www.saintpatrickdc.org/ss/1113.htm#nich

Nicholas I, Saint, Pope
- Short biographical entry in the Columbia Encyclopedia, 6th ed.
- http://www.bartleby.com/65/ni/Nichls1.html

Pope St. Nicholas I
- Biography of this pope, d. 867, who upheld the authority of the papacy and the inviolability of marriage. In the Catholic Encyclopedia.
- http://www.newadvent.org/cathen/11054a.htm

St. Nicholas I, the Great
- Profile of this pope.
- http://www.cfpeople.org/Books/Pope/POPEp106.htm