Officers believe the fire, in a store building at the rear of a shop in Brighton Road, Worthing, was started deliberately.
West Sussex Fire and Rescue Service received multiple calls shortly before 5am on Sunday.
A Sussex Police spokesman said: “Emergency services attended and two men from the Brighton Road property were treated at hospital for the effects of smoke inhalation.
“Statements were taken, the fire is being treated as deliberate and is currently under active investigation.
“Police forensic experts are liaising with fire service investigators, house-to-house enquiries have been made and enquiries continue to locate any CCTV.
“A van parked outside the Brighton Road property is believed to have been damaged before the fire and this forms part of the investigation.”
The fire service confirmed that its officers ‘have been assisting Sussex Police with the investigation’.
Anyone with information is asked to contact police on 101, quoting serial 206 of November 28.