Abstract Magnetic interaction in solvent-free DPPH and DPPH–solvent complexes has been studied by means of measurements of the magnetic susceptibility, the proton magnetic resonance, and the low-field ESR at very low temperatures. The susceptibility of solvent-free DPPH shows a round maximum at 11 K, and then it decreases with a lowering of the temperature, no ordered state being observed. The DPPH–benzene (1 : 1) complex shows a round maximum of susceptibility at 0.65 K and antiferromagnetic ordering at about 0.40 K. The DPPH–CCl4 (4 : 1) complex shows a curious temperature dependence of the susceptibility, which is interpreted in terms of a two-magnetic sublattice model; one sublattice is in the singlet spin state, while the other is in the paramagnetic state at low temperatures except below 0.45 K. Pair interaction is dominant both solvent-free DPPH and in solvent complexes of DPPH. A theoretical approach to the ordered state of DPPH–benzene is given by taking the inter-pair interaction into account.