It is shown that large errors due to long wavelength harmonics can occur in the determination of integrated intensities and halfwidths with a double crystal spectrometer. For example, in measuring the integrated intensity of the (333) reflection of silicon with MoK&agr; radiation, an error of a factor of 4 is possible due to 3&lgr; radiation from the white spectrum which is simultaneously diffracted by the (111) planes. The amount of error is dependent upon the particular experimental arrangement (tube voltage and allowed horizontal divergence being the most important factors), and can be reduced, and in most cases eliminated, by use of an appropriate filter.