Camphor-10-sulfonic acid (CSA) 1 is routinely used for calibrating circular dichroism (CD) instruments. However, CSA is hygroscopic, so an accurate concentration can be obtained only by measuring its absorbance peak at 285 nm. Hence, calibrations are highly dependent on having a cor-rect value for the extinction coeYcient for this compound. Following an extensive search of the literature, the ori-gins of the commonly accepted value for the molar extinc-tion coeYcient for CSA of 34.5 M ¡1 cm ¡1 at 285 nm can be ascribed to Chen and Yang [1], who cited Lowry and French [2], although Lowry and French reported the value at 284 nm to be 35 M ¡1 cm ¡1 , following measure-ments at 286 and 292 nm [3]. (There was actually no data point at the peak maximum.) The value of 34.52 M ¡1 cm ¡1 at 285 nm is reported in instrument manuals without refer-ence, usually quoted in the form of a 1 mg/ml solution producing an absorbance of 0.743 in a 5-mm cell (also see [4]). Recently, more stringent methods of standardizing the calibration of conventional and synchrotron radiation CD instruments have been developed [5]. Considering that CSA is the most widely used standard for calibration and cross-correlation for both types of CD instruments, we decided to re-examine the measurement of the extinction coeYcient of CSA, concentrating on reproducibility (error levels) and measurements at the actual peak maximum. Materials and methods CSA of more than 99% purity was obtained from Sigma–Aldrich. Standardized 0.5-M KOH solution and neutral red pH indicator were purchased from BDH Chemicals. In the experiment, 25 ml of a nominally (based on gravimetric measurements) 0.5-M aqueous solution of CSA were titrated against 0.5 M KOH using a 50-ml Goldline glassware burette (H.J. Elliot). Three diVerent batches of CSA and three batches of KOH were used, with each titration being repeated Wve times. The extinction coeYcient was determined at the absorption peak (285 nm) using a Cary 3 UV/Vis spectrophotometer calibrated for baseline stability, wavelength accuracy, and photometric accuracy within an absorbance range of 0.2–1.0. A number of dilutions and diVerent cuvettes of 1-, 5-, and 10-mm path lengths were used to eliminate systematic errors. The cuvettes were calibrated using the chromate method (A.J. Miles et al., submitted). Dilutions were carried out using A-type volumetric Xasks and calibrated pipettes. Each measurement was repeated Wve times. Fig. 1 shows the calibration curve obtained for the repeated measure-ments. (B.A. Wallace). 1 Abbreviations used: CSA, camphor-10-sulfonic acid; CD, circular dichroism.