We report measurements of the electrical resistivity, the Seebeck-coefficient and the magnetic susceptibility of samples (La 1− x Sr x ) 2NiO 4+δ in the concentration range 0 ≤ χ ≤ 0.5. All compounds exhibit semiconducting behaviour at low temperatures with thermally activated or variable range hopping type of conductivity. The comparison of the temperature dependence of the conductivity and the Seebeck-coefficient reveals the polaron character of the charge carriers. From the susceptibility measurements at high temperatures we derive an exchange interaction J af≈ 140 K for the NiO 2-planes in La 2NiO 4. We develop a magnetic phase diagram for (La 1− x Sr x ) 2NiO 4 with an antiferromagnetic range for x≤0.04 and a spin glass range for x > 0.04. The results are discussed in comparison with those of the high temperature superconducting system (La 1− x Sr x ) 2CuO 4.