Superconducting La 2 CuO 4 - δ F y samples were synthesized by heating La 2 CuO 4 together with a fluorinating reagent, NH 4 F, at 240°C in flowing oxygen gas. Bulk superconductivity with T c = 42 K in the samples was confirmed by DC magnetic susceptibility measurements. The enhancement in the orthorhombicity of the K 2 NiF 4 phase (a = 5.319 , b = 5.442 , c = 13.20 ) was observed. The lower and upper critical fields, H c l (0) and H c 2 (0), for the La 2 CuO 4 - δ F y were roughly determined to be 98 Oe and 600 kOe, respectively.