The bromoil process is based on the fact that when a silver gelatin print is chemically treated with a solution containing a dichromate salt, the gelatin hardens in proportion to the amount of silver present. It is also based on the fact that oil and water repel one another as in lithography.

To make a bromoil print, a silver gelatin bromide print was made by enlargement. The print was treated with a bleaching solution containing potassium dichromate which hardened the gelatin in proportion to the amount of silver present in the print. Simultaneously the silver was oxidized to its colorless ionic state, a process known as “bleaching.” The print was fixed and washed, resulting in a clear gelatin relief image. The print was then immersed in water and kept damp, swelling the unhardened gelatin in the highlights and partially swelling the mid-tones. A greasy lithographic ink was applied to the print with a special brush. The greasy ink was repelled by the water soaked highlights and attached to the hardened shadow areas. The mid-tones absorbed the ink in proportion to the amount of hardening.

Bromoil was an improvement over the oil process in which paper coated with dichromated gelatin was contact printed.  The name Bromoil is derived from the combination of bromide and oil.  While the process was intended as an independent process, the bromoil transfer process became a popular derivative. Oil, bromoil and bromoil transfer were popular with art photographers working within the Pictorialist movement.