In PG there were no diphthongs. There was just a sequence of two separate vowels. Diphthongs appeared in OE: some (usually long diphthongs) – as a result of merging of two vowels:
| Sounds | Diphth. | Gothic | OE |
| a + u à | ea: | auso | eare (ear) |
| e + u à | eo: | þeudans | þēoden (king) |
| (i + u)à | (io:) (dialectal variant) | diups | dīop (deep) |
others (usually short diphthongs) – as a result of the influence of the succeeding and preceding consonants (breaking of [æ, e]):
| Monoph. | Diphth. | Influence | Gothic | OE |
| æ à | ea | before l | alls | eall (all) |
| æ à | ea | before h | ahtau | eahta (eight) |
| e à | eo | before r | herza | heorte (heart) |
| æ à | ea | after sk’/k’ | skadus | sceadu (shade) |
| æ: à | ea: | after j | jâr | ζēar (year) |