Sm(OH)2NO3, was prepared by hydrolysis of the corresponding nitrate, leading to a product which is insoluble in water. IR-spectroscopy showed a comparison with the analogous Gd-compound. The temperature of decomposition to SmONO3 is below 573 K. The structure was refined by the Rietveld technique in space group P21, (Z=2), a=6.3852(3) Å, b=3.7784(2) Å, c=7.7402(3) Å, β=97.572(3)°, V=185.11 Å3, Rp=4.9, Rwp=6.3, RB=4.0. The compound is isotypic with Ln(OH)2NO3 (Ln=Pr,Nd,Gd). Infrared spectra and thermal decomposition data are also given for the Gd phase.