An examination of the theoretical relationship between absorption and crystal response in a Stark‐effect microwave spectrograph indicates that a direct proportionality exists between the absorption coefficient and the component of crystal current at the modulation frequency, provided that certain reasonable conditions are fulfilled.An experimental method is described which utilizes this principle to obtain measurements of the ratio of one absorption coefficient to another. This method involves a matching of the modulation frequency component of crystal current against an adjustable reference signal. These measurements may be used to determine the energy separation of the lower states of the lines observed, thus yielding microwave data on vibrational or torsional energy levels. Line breadth and saturation can be measured equally well with minor modifications in method.This experimental method was tested by measuring the known ratios of absorption coefficients of the carbonyl sulfide lines near 24 000 mc which are assigned to various isotopic species. The range of deviation of measured ratio from known ratio was 25 percent, and this discrepancy is attributed to reflections in the microwave transmission line, particularly at the ends of the Stark electrode. The determination of molecular vibrational or torsional energy levels is discussed.