Phosphorus was a lightly built, unprepossessing horse; viewing him immediately after his win at Epsom, the Farmer's Magazine praised his trainer for the colt's excellent condition, but could find little to compliment in his appearance.

Day was also the jockey when Phosphorus returned to Newmarket in May against ten opponents in the Rowley Mile Plate.

Phosphorus was moved to a base at Epsom to complete his preparation for the Derby, but on his first gallop after arriving he sprained a foreleg.

[4] On the morning of the Derby Phosphorus was walked for four hours and, having returned sound, was cleared by his owner to run, with the ride going to George Edwards.

[5] News of Phosphorus' problems led to his price for the Derby drifting, despite the fact that he had been "tipped" as a possible winner in a poem published in Bell's Life magazine which ended: On 25 May Phosphorus started at odds of 40/1 in a field of seventeen runners for the Derby, with Rat-trap being made the 6/4 favourite.

The pace was exceptionally fast and most of the runners were struggling well before the turn into the straight, by which point Phosphorus and the second favourite Caravan had moved up to dispute the lead.

The two colts pulled clear of the rest of the runners and raced side-by-side throughout the closing stages in a "magnificent" contest.