Indo-European Phonetics
Diphthongs
Chart of phonemic correspondences

IE ai ei oi au eu ou
Celtic ai, é (5) é oi, ai (5) au, ó (5) ou (5) ou (5)
Italic ai, ae (2) ei, í (2) oi, ú (2) au, o (2) ou, ú (2) ú (2)
Venetic ai ei oi au eu ou
Hellenic ai ei oi au eu ou
Illyrian ai ei oi au eu ou
Thracian ai ei       au
Armenian ay   é  aw oy oy
Germanic ai, é (3) í  ai au, ó (3) iu (3) au, ó
Slavic e. i e., i (4) u u u
Baltic ie ei (1) ie, ai au jau au
Indo-Iranian ai (6) ai (6) ai (6) au (6) au (6) au (6)
Anatolian ai ei ai  au au au
Tocharic ay ey oy aw ew ow

Transcription of symbols:
á, é, í, ó, ú - long [a], [e], [i], [o], [u]
e. - Slavic open [e]
ü - pronounced like a French sound in rue
 

Notes:

1. IE *ei > Old Prussian ei, eu, Lithuanian ie, ei, ai
2. IE diphthongs have various reflections in Italic: in Osco-Umbrian subgroup they disappeared in Umbrian (*ai, *oi > e, *ei > i, au > o) but were preserved in Oscan. In the Archaic Latin language, diphthongs ai, ei, oi were used, but later they were transferred into ae, í and ú respectively.
3. IE diphthongs were preserved in the Common Germanic language (except *ei > *í), but later separate Germanic tongues show a great variety of forms descended from diphthongs. For example:
        Germanic *ai > Old English & Old Norse á (stán 'a stone'), Swedish é (stén 'a stone'), Old High German ei (steinn), Gothic ai (stains)
        Germanic *iu/*eu > Gothic iu (biudan 'to offer'), Old English éo (béodan), Old High German io (biotan), Old Norse jó / jú (bjúdo)
4. IE *oi > Slavic e. everywhere except the final position in the word where it > i
5. IE diphthongs had different historical routes in Celtic languages:
        IE *au > Gaulish au, Old Irish ó
        IE *eu, ou > Common Celtic *ou > Old Irish & Gaulish ó, Welsh & Breton ü
        IE *ai > Gaulish ai, é, Old Irish ai, oi
        IE *oi > Gaulish oi, Old Irish oi, ai
6. Indo-Iranian *ai and *au were written as e and o in Indic languages, though functionally they remained diphthongs.
 

Examples:

1. IE *ai
*daiwér- 'brother-in-law'
*kaiko- 'blind'

2. IE *ei
*deiwo- 'deity'
*lei- 'to pour'

3. IE *oi
*oi-n-, *oi-k- 'one'

4. IE *au
*aus- 'an ear'
*nau- 'a vessel, a ship'

5. IE *eu
*bheud- 'to be awaken'
*dheu- 'to blow'

6. IE *ou
*gwou- 'bull, cow'
*oug- 'cold'
 

Linguistic comment

A diphthong is a combination of a vowel and a sonant which is pronounced as one sound: in English, there are numerous diphthongs like in sound, ride, spoil. Indo-European linguistics suggests that in the Proto-language there were in fact two different sets of diphthongs: those with a short vowel (ai, au) and with a long one (ái, áu). However, they coincided in the majority of languages - except maybe the Indo-Iranian group which still preserves long diphthongs in the ablaut series in the root: Sanskrit deva- 'god' from Indo-Iranian *daiwa- < IE *deiwo-, but Sanskrit gaus 'a bull' from Indo-Iranian *gáus from IE *gwóus.

The most progressive languages usually tend to turn diphthongs into the single vowels - for example, of the Italic group, the most archaic Oscan languages kept them, but Latin dropped. Diphthongs were quite unstable in any language, so sometimes we cannot even identify the general direction of language changes - diphthongs disappear and emerge once again (like in English). Several modern languages of the family even have triphthongs (three components included in one sound) - like in English sour - but obviously the Proto-language did not have such.