Three-dimensional structures of the title alkaloids are proposed on the basis of: (1) showing that the relative configuration of C-I, C-4, and C-IO of the AB and A'B' quinolizidine systems is the same as the relative configuration of these centers in deoxynupharidine; (2) demonstrating that the carbon atom of the CHzS group is attached to the A'B' quinolizidine system in both alkaloids in an equatorial fashion; and (3) showing that the sulfur atom of the CH2S group is attached in an equatorial fashion to the AB quinolizidine system of thiobinupharidine but is attached in an axial fashion to the AB quinolizidine system of thionuphlutine-B. Evidence for the sulfur atom stereochemistry in the AB quinolizidine systems came from an nmr determination of the steric course of sodium borodeuteride reduction of 6,6'-dihydroxythiobinupharidine and 6,6'-dihydroxythionuphlutine-B. 6-Hydroxythiobinupharidine was isolated from Nuphar luteum. The latter alkaloid, 6,6'-dihydroxythiobinupharidine, 6,6'-dihydroxythionuphlutine-B, and two newly synthesized model compounds, 7a-methylthio-7-epideoxynupharidin-6P-01 and 7P-methylthiodeoxynupharidin-6a-01, in acid solution showed absorption maxima in the region of 290-295 nm (E 1500-3200). Reduction of all of these compounds with sodium borodeuteride resulted in the incorporation of deuterium stereospecifically. The uv and reduction results are interpreted in terms of sulfur-immonium ion interaction.