Heat capacity measurements from 150 to 300 K were carried out on a single crystal of La2CuO4+x synthesized by subjecting an La2CuO4 crystal to 3 kbar oxygen pressure at 575 °C. The data reveal three small (about 1%) anomalies at temperatures (T) of 206, 222 and 259 K. The first two are tentatively attributed to CuO inclusions in the crystal. The third is observed on warming, but not on cooling, and is attributed to the previously documented first-order transition from the orthorhombically distorted K2NiF4 structure to a low T mixture of nearly stoichiometric La2CuO4 and oxygen-rich superconducting La2CuO4+y (y>x). The size of the anomaly at 259 K is about one-seventh of that observed previously for a single crystal of La2CuO4 at the second-order tetragonal-to-orthorhombic phase transition temperature of about 530 K. Magnetization measurements from 5 to 300 K and from 50 G to 50 kG are also reported for the La2CuO4+x crystal. The normal state magnetic susceptibility χ(T) is quite anisotropic, with χ(T) for H perpendicular to the CuO2 layers (χc) in good agreement with previous data on a different, but similarly prepared, crystal. The anisotropy in χ is nearly independent of T from 40 to 300 K and the magnitude of χc-χab per CuO2 layer is very similar to that at high T in YBa2Cu3O6.1, La2CuO4, Sr2CuO2Cl2 and La2-xMxCuO4 (MSr, Ba). © 1992.