Studies were conducted to determine the effect of temperature and wavelength on the absorbance of alkaline solutions of picric acid in the presence and absence of creatinine. Absorbance values of an alkaline solution of picric acid were found to be influenced by temperature. At wavelength settings between 475 and 520 nm, absorbance values increased as the temperature increased. The magnitude of the thermochromic response (temperature-induced increase in absorbance) was found to be a function of wavelength: At 490 nm, the response was about three times greater than it was at 520 nm and about fifteen times greater than it was at 520 nm. Other experiments demonstrated that the response was: quantitatively related to picric acid concentration, reversible, rapid, and independent of creatinine concentration.