The present disclosure relates to a method and apparatus for testing materials. More particularly, but not exclusively, this invention concerns a method and apparatus for testing materials using infrared spectrometry. The invention also concerns the calibration of an infrared spectrometer for use in testing materials using infrared spectrometry. The invention provides a method of calibrating an infrared spectrometer for testing composites in the aerospace industry, comprising the steps of, selecting a plurality of variables which have the potential to influence the physical characteristics of a composite used in the aerospace industry, selecting a plurality of values for each variable and inputting the variable and values into a design of experiments model, thereby obtaining a sample test matrix.