The charge transfer salt β′−(ET) 2 SF 5 CF 2 SO 3, which has previously been considered a spin-Peierls material with a T SP∼ 33 K, is examined using high-resolution high-field sub-millimeter/millimeter wave electron spin resonance (ESR), and nuclear magnetic resonance (NMR) techniques. A peak in the nuclear spin-lattice relaxation behavior in fields of 8 T, accompanied by a broadening and paramagnetic shift of the resonance line, indicates a phase transition at T c∼ 20 K. A pronounced change in the high-field ESR excitation spectra at∼ 24 T, observed at T c∼ 20 K, may indicate the onset of antiferromagnetic (AFM) correlations of the low-temperature phase in β′−(ET) 2 SF 5 CF 2 SO 3. Peculiarities of the low-temperature magnetic and resonance properties of β′−(ET) 2 SF 5 CF 2 SO 3 are discussed.