Rylan Clark-Neal has pulled out of hosting BBC Four's coverage of the Eurovision semi-finals tonight due to illness.
Cox, who previously worked on Eurovision in 2011 and 2012, will join Mills and new co-host Chelcee Grimes at the BBC's studios in London to commentate on proceedings in Rotterdam tonight and for the second semi on Thursday.
"I'm really excited, I've got a new little job this week," she revealed on her Radio 2 show on Monday. "I will be co-hosting the Eurovision semi-finals on BBC Four tomorrow night and Thursday night.
"Rylan is unwell, so obvs we wish him better. So I'll be stepping into Rylan's sparkly tux tomorrow and Thursday, 8pm, BBC Four, all the action from the Eurovision semi-finals.
"I'll also be rekindling the flames of fun with my old Eurovision mucker Scott Mills, and we'll be joined by Chelcee Grimes and many guests as well."
Sixteen countries - including one-time favourites Malta, as well as the likes of Ireland, Sweden, Australia and Russia - will take to the stage tonight to compete for 10 available spots in Saturday's grand final.