We review the dc transport properties of mixed valent compounds SmB6and gold‐SmS and find the character of the ground state, metallic or insulating, to be an open question. The room temperature optical properties of SmB6and gold‐SmS display ordinary metallic behavior at high energy, but are very nonmetallic at low energy in thatE1becomes positive like an insulator from about 0.1 eV down to the lowest energy measured. We show that these room temperature optical properties are consistent with a ’’bad‐metal’’ model having either a sharp 4fresonance at the Fermi level, or a collective low frequency density fluctuation, and extract from the data a value ∼.08 eV for the 4f‐5dmixing matrix element. But we point out that the possibility of an insulating ground state remains.