Apparent molar volumes have been determined using a high-pressure vibrating-tube densimeter for aqueous solutions of glycolic acid (HGly = HOCH2COOH) and tartaric acid (H(2)Tar = HOOCCH(OH)CH(OH)-COOH) at temperatures from 25 degrees C to 350 degrees C and pressures as high as 20 MPa. The resulting standard partial molar volumes V-2(o)(HGly,aq) are relatively independent of temperature until 315 T, at which point V-2(o)(HGly,aq) deviates sharply toward negative values. This suggests that the Krichevskii parameter, A(Kr), = lim(x(2) -> 0) (partial derivative p/partial derivative x(2))(T,V)(c), which describes the discontinuities in standard partial molar properties at the critical point of water, is negative. Almost all aqueous nonelectrolytes are characterized by positive Krichevskii parameters. This is the first negative value reported for any organic molecule that is not an ion or zwitterion and only the third ever observed directly for a neutral species (the others are B(OH)(3) and H3PO4). The standard partial molar volumes for H(2)Tar(aq) are also relatively independent of temperature until 275 degrees C, suggesting a similar behavior. However, the onset of thermal decomposition prevented measurements at temperatures above 300 degrees C.