We describe a method for precisely measuring the solubility of proteins in aqueous solution using laser confocal differential interference contrast microscopy. The method is based on the in situ observation of single steps on a protein crystal surface which allows a fast and precise determination of solubility as a function of temperature. To demonstrate the effectiveness of this novel approach the solubility dependence on temperature of glucose isomerase and hen egg white lysozyme was determined with a precision of -/+ 0.5 degrees C or smaller. It was found that a small amount of impurities did not significantly change the obtained solubility data. Numerical values for enthalpies and entropies of crystallization were calculated and they compare well to previously reported values but the experimental errors were significantly reduced. (C) 2009 Elsevier B.V. All rights reserved.