The magnesium molecules Mg-2, Mg-3, Mg-4, and Mg-5 have been isolated in rare-gas matrixes at 8 K and characterized for the first time by Raman spectroscopy. The vibrational frequencies were compared with those obtained by density functional theory calculations. The fundamental frequency of the dimer was observed at 97 cm(-1) (Ar matrix), and a bond length of d(Mg-Mg) = 340.8 pm was calculated. The Mg-3 cluster possesses a triangular structure (D-3h) with calculated d(Mg-Mg) = 317.8 pm. For tetramagnesium, a tetrahedral structure (T-d) with d(Mg-Mg) = 301.3 pm is predicted. The Mg-5 molecule possesses a trigonal-bipyramidal structure (D-3h) with d(Mg-Mg) = 293.1 and 322.5 pm, respectively. The red tetramagnesium cluster is an example of a matrix-isolated magic number cluster.