Abstract 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