Problems associated with the molecular interpretation of mesomorphic behavior within a given or between different homologous series and possible effects of impurities on mesomorphic properties are considered. The temperatures and enthalpies of phase transitions were determined for the first twenty cholesteryl esters and compared with the results of corresponding thiocholesteryl esters. The investigated Compounds had a purity of 99 mole percent or better. With two exceptions, the thiocholesterols exhibited higher clearing temperatures, but generally lower heats of clearing. Smectic phases were observed for choleateryl octanoate and higher members, while for thiocholesterol these phases started with the heptanoate. The study of ω-phenyl cholesteryl esters (first 8 members) showed that all members exhibit cholesteric mesophases except the acetate. The values of heats of clearing lie on two well separated curves, one for odd and the other one for even values of chain length. The corresponding ω-phenyl thiocholesteryl esters exhibit higher clearing points for odd values of acid chain length. For even chain length only the octanoate exhibited a mesomorphic phase. Because of the lack of knowledge about molecular arrangement and interactions in cholesteric mesophases, the effects of the sulfur substitutions on mesomorphic behavior cannot be explained.