Encyclopædia Britannica: Cyril
- Profile of the missionary. In the 11th edition (1911). Some scanner errors.
- http://encyclopedia.jrank.org/CRE_DAH/CYRIL_827_869_.html

The Ecole Glossary: Cyril
- Short biography of St. Cyril, also known as Constantine the Philosopher, inventor of the glagolithic (Cyrillic) alphabet, by Karen Rae Keck.
- http://www2.evansville.edu/ecoleweb/glossary/cyrilaps.html

Patron Saints Index: Cyril
- Illustrated profile.
- http://www.catholic-forum.com/saints/saintc08.htm