The adduct (C5H5)3SmIII(NCCH3) turns out to crystallize not only in the already reported [3] orthorhombic form 1a (space group Pnma), but also as a triclinic modification 2b with space group P1BAR and a 854.6(3), b 865.9(3), and c 1094.3(4) pm; alpha-68.17(4), beta-82.49(3) and gamma-85.63(3)-degrees; final R-value: 0.033 (R(w) = 0.032). 2b represents a structural alternative which appears to be more readily made, but somewhat less useful, e.g. for spectroscopic studies of spatially oriented single crystals.