Police were called to Glanmor Crescent, Barry, at about 21:15 BST on Thursday.
A 54-year-old woman was taken to hospital with burns where she remains in a serious condition.
A South Wales Police spokesman said the arrested man from Penarth was believed to be known to the victim.
"A crime scene is being preserved at the address and a police cordon has been set up," he added.
The investigation is ongoing and an appeal has been made for witnesses.
A fire crew from Barry and an ambulance were also in attendance.
A Welsh Ambulance Service spokesperson said: "We were called at about 21:35 BST on Thursday, 4 August to reports of a fire at an address in Barry.
"A crew in an emergency ambulance attended the scene and a woman was taken to the University Hospital of Wales."
She was later transferred to Morrison Hospital.
A 63-year-old man has been arrested in connection with a serious assault in the Vale of Glamorgan which left a woman with severe burns.