From extracts of Oncinotis tenuiloba STAFF, two novel polyamine alkaloids, oncinotin-11-one (5) and oncinotin-12-one (6), were isolated. Peracetylation of 6 provided the N-acetyl derivative 11 as well as N,N'-diacetylinandenin-10-en-12-one (12) due to a beta-elimination-type side reaction resulting in ring enlargement of 11 (Scheme 1). Deuteration of 12 yielded 13, showing the same retention time as N,N'-diacetylinandenin-12-one (14), when co-HPLC was performed together with different keto-isomeric N,N'-diacetylinandeninones. Structure elucidation was extended by Schmidt degradation of 6 and N,N'-diacetyl(10,11-H-2(2))inandenin-12-one (13); the degradation products were identified by GC and ESI-MS. The structure of 5 was proposed on the basis of spectroscopic means. Comparison of the spectroscopic data of 5 with those obtained from synthetic material as well as co-HPLC of the N-acetyl derivative 20 together with the corresponding synthetic compound revealed the identity of the substances and confirmed the structure of 5. Additionally, oncinotine (2) and neooncinotine (3) were isolated, separated, and identified with authentic samples by co-HPLC of their N-acetyl derivatives 8 and 9, respectively.