Ultraviolet spectrophotometric investigations have been carried out the systems of monoalkylbenzene with iodine in carbon tetrachloride. The results reveal the formation of one to one molecular complexes. The equilibrium constants for these complexes of representative monosubstituted benzene reveal the following order of increasing stability: benzene < methyl- < ethyl- < n-propyl-benzene. The value of ${Delta}H$, ${Delta}G$, and ${Delta}S$ for interaction of a number of monoalkyl substituted benzene with iodine has been determinated. In general, as ΛH becomes increasingly negative, corresponding decreases in ${Delta}G$ and ${Delta}S$ values are observed, and these variation are linear. The thermodynamic constants become increasingly negative with increasing monoalkyl substitution of the aromatic donor nucleus. The complex bond is therefore weak, and its formation is accompanied by relatively small entropy changes.