Abstract-Fragmentation studies of the mass spectra of 24 high purity compounds of importance in a study of fossil fuels are described. The substances include eleven I-(2-thienyl)-l-thia-, sixl-(2-thienyl)-3-thia, and seveni-(2-thienyl)-4-thiaalkanes in the molecular weight range 158 to 270. Rearrangement peaks are prominent in al1 spectra and form the base peaks for the 1-thia-and 4-thia-compounds. The 3-thia-compounds are somewhat perverse in exhibiting weaker rearrangement peaks, and large ion intensities, including base peaks, at masses typical of aikyl fragments, alkenyl fragments, or ions more characteristic of the thiaalkanes (aliphatic sulfides). The major fragment ions are discussed and their use in differentiating subclasses is described. Low ionization voltage data is presented in support of suggested mechanisms of fragmentation.