The isolation of three new N-methylated piperidine-3-ol alkaloids from the bark of Prosopis affinis Spreng. (Ñandubay) is described. Together with MS and IR data, results from 1D and 2D NMR experiments allowed for the identification of N-methyl-2-isocassine, N-;methyl-6-isocassine, and N-methyl-6- isocarnavaline. Inspection of 1D-NOESY spectra and coupling constant data revealed that the heterocyclic moiety of the alkaloids is in fast conformational equilibrium in solution, and this behavior had to be taken into account in order to determine the relative configuration of the chiral centers of the piperidine rings.