Diophon's dam Donetta was a durable racemare who competed until the age before becoming a very successful broodmare, with her other foals including Diadem, Dionysos (Irish St Leger) and Diadumenos (Jubilee Handicap).

[6] Despite conceding weight to his seven opponents, Diophon started favourite for the Hopeful Stakes at Newmarket in early October but sustained his first defeat as he was beaten a head into second place by the previously unraced Woodend.

[7] Two weeks later, in "dismal weather conditions", Diophon started 2/1[5] favourite in an eleven-runner field for the Middle Park Plate over six furlongs at the same track.

[8][9] Although Diophon was considered a leading contender for the following year's British Classic Races he was regarded as being inferior to his female stablemate Mumtaz Mahal.

In a very close finish, which saw the leaders racing up opposite sides of the wide Newmarket track, he was adjudged to have won by a head from Bright Knight with Green Fire a neck away in third.

[14] On 4 May 1925 Diophon contested the Lingfield Park Spring Stakes and finished second, beaten a head by Sansovino, with the Argentinian colt El Cacique three lengths back in third.

[15] At Newbury Racecourse on 10 June he was assigned a weight of 140 pounds for the Royal Plate and won from Glommen and Invershin despite struggling to obtain a clear run in the straight.