Fluorescence intensity, sensitivity, and the effect of diverse ions are discussed in relation to chemical equilibrium and the general equation for fluorescence. High sensitivity is the common denominator in eliminating or reducing all types of interference and the general equation is the key for quickly selecting conditions that give maximum sensitivity. With a transmission-type fluorometer, experimental fluorescence intensities adhere to the general equation over a very wide range of light absorption. If the instrument has a vertical axis, solution depth can be adjusted so that the data fall on that portion of the theoretical curve which is essentially a straight line. When the fluorescence wavelength selected for measurement is not absorbed by the solution, the general equation reduces to a much simpler expression, and then, under proper circumstances, fluorescence values can be used for calculations as confidently and as easily as absorbance values.