I'm working from Tim Rudman's toning book and am playing with the FSA toner. In it he lists thiourea dioxide to be used in the toning bath. Now I have thiourea that comes from TechChem down in Missouri and called them to see what flavor of thiourea I have. Bob called back and he found something odd in the CAS listings. TechChem's thiourea is CAS# 62-56-6 Thiourea dioxide has TWO CAS #'s which he says is "unheard of". CAS# 4189-44-0 and CAS# 1758-73-2 Since Tim has been known to hang around these parts and others here may be more familiar with the CAS catalog system, I'm wondering if these 3 compounds are interchangeable or what? Thanks Eric