The alkaloidal content of Senecio Jacobea L. was first closely examined by Manske, who described a crystalline alkaloid jacobine (C₁₈H₂₇O₅N) yielding retronecine (C₈H₁₃O₂N) and jaconecic acid (C₁₀H₁₆O₆, with reserve) on hydrolysis. However, hydrolysis would require three water molecules (unusual for Senecio alkaloids, which need at most two for ester and lactone groups), and Manske's analytical results did not agree with the formulae. We previously found two alkaloids in S. Jacobea differing in rotation; one was isolated by Hosking and Brandt (C₁₈H₂₅O₆N, unnamed). Later, material collected in August 1935 yielded only one alkaloid (identical to that of Hosking and Brandt), which we retain the name jacobine as the principal alkaloid. Our analyses support the hydrolysis equation C₁₈H₂₅O₆N + 2H₂O = C₈H₁₃O₂N + C₁₀H₁₆O₆: the basic product is retronecine, and jaconecic acid (isomeric with other Senecio acids) contains three (or four) C-methyl groups and is likely an acyclic hydroxy-ketonic acid. From 1934 July material, jacobine was mixed with an isomeric alkaloid C₁₈H₂₃O₅N, named jacodine (higher levorotation, lower solubility in ethyl acetate, more soluble nitrate in alcohol), explaining Manske's impure sample. From the mother-liquors, we isolated jaconine (C₁₈H₂₃O₈N·H₂O), very soluble in alcohol, absent in later material. Senecio Jacobea from New Zealand had the same alkaloidal content (0.06%) and mixture (jacobine + jacodine) as Scottish samples; greater poisoning incidence is due to more abundant plants. Jacobine is also the principal alkaloid in S. cineraria (a Mediterranean species hybridizing with S. Jacobea), though Manske found it practically devoid of alkaloid. We suggest aureine (from S. aureus) may be identical with senecionine (Manske failed to obtain pure), and the acid C₁₀H₁₆O₄ from S. mikanoides is dimethylacrylic acid (previously isolated and identified).