A procedure is given for correcting optical absorbance measurements made at variable pressure with a le Noble-Schlott ("pillbox") cell for the inner sleeve wall thickness. With this technique, the molar volume change for the acid ionization of aqueous [Cr(Hedta)OH2] was found to be +5.1 +/- 0.6 cm(3) mol(-1) (0-200 MPa, 25.0 degrees C, ionic strength 1.0 mol L(-1) HClO4/NaClO4), an anomalous positive value which implies a change from quinquedentate to predominantly sexidentate edta and expulsion of the coordinated water on ionization. For thiocyanate substitution into labile [Cr(Hedta)OH2], high pressure stopped-now measurements gave the volume of activation as -7.8 +/- 0.9 cm(3) mol(-1) and the volume of reaction as +3 +/- 2 cm(3) mol(-1), while for the reaction of [Cr(edta)](-) with NCS- the activation volume is -13.6 +/- 0.6 cm(3) mol(-1) (same conditions). These and other data support the notion that the anomalous substitutional lability of Cr-III(edta) complexes relative to typical Cr-III species is due to activation by transient chelation of the pendant arm of quinquedentate edta.