The Thiourea Toner is actually a sulfide toner. In alkaline solution, thiourea (thiocarbamide) reacts to yield sulfide ions, the active ingredient in the toning bath. The advantage of this process is the avoidance of the foul smelling sodium sulfide bath.

To obtain beautiful rich brown, sepia-like tones, the print is first bleached using a ferricyanide-bromide bath then redeveloped in a sulfide bath. In this process, the silver metal in the print is first converted to silver bromide then to brown silver sulfide. The resulting print can give the impression of a print made in the late 1800's.