Magnetic properties of two nitronyl nitroxide (4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide) radicals which show nearly isotropic S=1/2 alternating linear chains are investigated down to 0.5 K and up to 9 T. Pentafluorophenyl nitronyl nitroxide at low temperatures shows an antiferromagnetic chain with alternation, while the material at room temperature shows a uniform chain structure. 2-Methyl nitronyl nitroxide, which contains an alternating chain with ferromagnetic and antiferromagnetic interactions, exhibits a three-dimensional phase transition at 1.3 K.