We have compared the photoisomerization properties of two cyanine analogs, 3,3’-diethyloxadicarbocyanine iodide (DODCI) and 3,3’-diethylthiadicarbocyanine iodide (DTDCI). These two molecules are structurally similar, differing only in the presence of oxygen or sulfur at two heteroatom sites. Measurement of the radiative and nonradiative population relaxation kinetics of these molecules reveals a difference in their equilibrium geometries despite their outward similarities. We relate this difference to the steric constraints imposed by the oxygen and sulfur heteroatoms and to the occurrence of an excited state barrier predicted by semiempirical calculations of the ground- and excited-state isomerization surfaces for these molecules.