The structure of the new bisbenzylisoquinoine alkaloid, thalfoetidine, was established by means of sodium in liquid ammonia cleavage of O-ethylthalfoetidine. The nonphenolic base of this degradation was identified as d-O-ethylarmepavine thus fixing the position of the phenolic hydroxyl group present in the molecule of thalfoetidine. Further a phenolic base was isolated from the above degradation. Its structure was established by synthesis as 1-(4-hydroxybenzyl)-2-methyl-5-hydroxy-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline. The two hydroxyl groups of this compound show the osition of the ether bridges in the molecule of thalfoetidine. The configuration of thalfoetidine and some related alkaloids is discussed. © 1971.