Experimental energy-loss data for 252Cf fission fragments in a number of gaseous compounds are presented. These data are compared with predicted energy loss data obtained by applying a modified semiempirical energy loss expression for the constituent elementary substances and by making use of the Bragg law of additivity of atomic stopping powers, yielding good agreement with experiment. Results obtained for energy loss in gaseous compounds consisting of both gaseous and solid components, stress the conclusion that within experimental uncertainties, the energy loss of fission fragments is independent of the physical state of the stopping medium. Thus, a reliable and accurate method for the estimation of fission fragment energy losses in composite systems of technological interest is provided. A commonly used method for estimating fission fragment ranges in composite materials was tested, and the deviation thus obtained, determined.