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