For polyimide thin films, the dielectric properties were investigated with the capacitance and optical methods. The dielectric constants of the 4,4'-oxydianiline (ODA)-based polyimide thin films varied from 2.49 to 3.10 and were in the following decreasing order: 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA)-ODA > 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA)-ODA > 4,4'-hexafluoroisopropylidene diphthalic dianhydride (6FDA)-ODA. According to the absorption of water, the diffusion coefficients in the films varied from 4.8 x 10(-10) to 7.2 x 10(-10) cm(2)/s and were in the following increasing order: BPDA-ODA < PMDA-ODA < 6FDA-ODA. The dielectric constants and diffusion coefficients of the polyimides were affected by the morphological structures, including the molecular packing order. However, because of the water uptake, the changes in the dielectric constants in the polyimide thin films varied from 0.49 to 1.01 and were in the following increasing order: BPDA-ODA < 6FDA-ODA < PMDA-ODA. Surprisingly, 6FDA-ODA with bulky hexafluoroisopropylidene groups showed less of a change in its dielectric constant than PMDA-ODA. The total water uptake for the polyimide thin films varied from 1.43 to 3.19 wt % and was in the following increasing order: BPDA-ODA < 6FDA-ODA < PMDA-ODA. This means that the changes in the dielectric constants in the polyimide thin films were significantly related to the morphological structure and hydrophobicity of hexafluoroisopropylidene groups. Therefore, the morphological structure and chemical affinity in the polyimide thin films were important factors in controlling the dielectric properties.