The effect of calcium chloride dihydrate on optical properties and the mass transport of methanol in poly(methyl methacrylate) (PMMA) has been investigated. The solvent transport was analyzed based on the Harmon model accounting for Case I, Case II, and anomalous diffusions. Both Case I and Case II transports satisfy the Arrhenius equation. Calcium chloride dihydrate depresses Case I diffusion and enhances Case II diffusion. From the mass transport data, the equilibrium solvent content of methanol with various concentrations of calcium chloride dihydrate in PMMA is obtained and satisfies the van't Hoff plot. A comparison of movement of sharp front and mass transport is made. The increase of UV transmittance of specimens treated with a mixture of methanol and calcium chloride dihydrate is attributed to the reduction of the carbonyl group. The critical solvent content to change transmittance from a slow increase to a fast increase occurs when the sharp fronts meet at mid-plane. The decrease of transmittance in UV range is due to void creation for a large amount of solvent. The visible light transmittance of the specimen with small solvent content is the same as that of the specimen without solvent treatment. The visible wavelength dependence of transmittance of specimens with large solvent content is attributed to the void of which the size is smaller than the wavelength of visible light. The void is enlarged during the mass uptake, then it is fully closed after desorption of a small solvent amount and shrinks to a small size after desorption for a large solvent content.